feat(discovery): AI tiebreak for ambiguous similarity matches

Ship 2 MR 4. Adds per-pair AI-backed classification for operator use
inside the existing Similar panel: an "AI?" button next to each
candidate asks Mistral whether the two queue rows refer to the same
underlying market. Result shown inline as a green "✓ same N%" or
grey "✗ diff N%" chip with the LLM's reason on hover.

No scraping — the classifier works from (name, city, year) alone,
which is enough for the common cases (same venue on two calendars,
typos, cross-year recurrence). Call is short (usually <3s) so the
handler is synchronous, 15s deadline.

Caching
- Migration 000020 adds similarity_ai_cache keyed on a content hash
  over (normalized_name|stadt|year) for both rows, sorted for
  symmetry. Survives queue row accept/reject because the hash is
  about markt-content, not queue-row lifecycle.
- enrich.SimilarityPairKey computes the key. Classify(a,b) and
  Classify(b,a) hit the same entry. Stadt casing drift doesn't
  invalidate.
- Repo methods GetSimilarityCache / SetSimilarityCache + corresponding
  mock hooks. DefaultSimilarityCacheTTL=30d.

Mistral integration
- enrich.MistralSimilarityClassifier reuses the same aiPass2
  interface as the enricher. English system prompt asks for
  JSON-only output with {same_market, confidence 0..1, reason}.
  Confidence clamped to [0,1] because models occasionally return
  1.2 or -0.1. Reason is short German justification.
- NoopSimilarityClassifier returns an error — callers must check
  ai.Enabled() before deciding which binding to pass.

Service.ClassifySimilarPair loads both rows, computes pair key,
cache-first, calls classifier on miss, writes cache, returns
verdict. Rejects self-comparison (pair-key collapses). Handler
POST /admin/discovery/queue/:aid/similar/:bid/classify.

UI: new AI? column inside the Similar panel. Per-candidate pending
state via Set<string>, disabled button while in-flight, inline
verdict chip after response. Tooltip shows the LLM's reason.

Tests: pair-key symmetry + differentiation + casing tolerance;
Mistral classifier happy path, clamping edge cases, error
propagation, bad-JSON handling, Noop rejection. Service tests:
happy path writes cache, cache-hit skips LLM, self-comparison
rejected, classifier errors don't poison the cache.

NewService signature grows by one param (sim enrich.
SimilarityClassifier). All 14 existing callers (routes.go + tests)
updated; tests pass nil.

backend/internal/domain/discovery/enrich/similarity.go

@@ -0,0 +1,181 @@
+package enrich
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+	"time"
+)
+
+// SimilarityRow carries the minimal identifying fields the classifier reads.
+// Callers adapt their domain row into this shape — the enrich package stays
+// free of a reverse import on discovery.
+type SimilarityRow struct {
+	NameNormalized string
+	Stadt          string
+	// Year is StartDatum.Year() (or 0 when unknown). Two rows differing only
+	// in year are almost always the same series in different editions —
+	// useful signal but not identity.
+	Year int
+	// Display fields — shown to the LLM verbatim so it can reason about
+	// casing / diacritics that NameNormalized strips.
+	Name string
+	// Quellen are passed in but not scraped — the classifier works from
+	// name/city/date alone. Included in the prompt only as metadata.
+	Quellen []string
+}
+
+// Verdict is the classifier's answer about one pair.
+type Verdict struct {
+	Same         bool      `json:"same"`
+	Confidence   float64   `json:"confidence"` // 0..1
+	Reason       string    `json:"reason"`
+	Model        string    `json:"model,omitempty"`
+	ClassifiedAt time.Time `json:"classified_at,omitempty"`
+}
+
+// SimilarityClassifier decides whether two queue rows refer to the same
+// underlying market. Implementations should be deterministic across calls
+// on the same input — callers cache the result.
+type SimilarityClassifier interface {
+	Classify(ctx context.Context, a, b SimilarityRow) (Verdict, error)
+}
+
+// NoopSimilarityClassifier returns a zero-confidence verdict without calling
+// anything. Used as a fallback when AI is disabled; callers should check
+// ai.Enabled() and fall back to this type rather than passing nil.
+type NoopSimilarityClassifier struct{}
+
+func (NoopSimilarityClassifier) Classify(_ context.Context, _, _ SimilarityRow) (Verdict, error) {
+	return Verdict{}, errors.New("similarity classifier not configured")
+}
+
+// SimilarityPairKey derives a content-hash cache key over the two rows'
+// identifying tuples. Ordering-independent: Classify(a,b) and Classify(b,a)
+// must hit the same cache entry.
+//
+// Uses NameNormalized + lowered Stadt + Year so the key survives casing
+// drift, whitespace, and umlaut normalisation (NameNormalized already did
+// the heavy lifting).
+func SimilarityPairKey(a, b SimilarityRow) string {
+	keyA := fmt.Sprintf("%s|%s|%d", a.NameNormalized, lowerASCII(a.Stadt), a.Year)
+	keyB := fmt.Sprintf("%s|%s|%d", b.NameNormalized, lowerASCII(b.Stadt), b.Year)
+	// Sort lexicographically for symmetry.
+	var raw string
+	if keyA <= keyB {
+		raw = keyA + "||" + keyB
+	} else {
+		raw = keyB + "||" + keyA
+	}
+	sum := sha256.Sum256([]byte(raw))
+	return hex.EncodeToString(sum[:])
+}
+
+// DefaultSimilarityCacheTTL: 30 days matches the enrichment cache TTL —
+// same reasoning (amortise cost across re-crawls, source edits eventually
+// propagate).
+const DefaultSimilarityCacheTTL = 30 * 24 * time.Hour
+
+// MistralSimilarityClassifier implements SimilarityClassifier by sending a
+// JSON-formatted comparison prompt to Mistral's chat endpoint.
+type MistralSimilarityClassifier struct {
+	Client aiPass2
+}
+
+// NewMistralSimilarityClassifier binds a Mistral ai.Client. client must be
+// non-nil; routes.go falls back to NoopSimilarityClassifier when AI is off.
+func NewMistralSimilarityClassifier(client aiPass2) *MistralSimilarityClassifier {
+	return &MistralSimilarityClassifier{Client: client}
+}
+
+// simResponse is the JSON shape we instruct Mistral to return. Confidence
+// must be parseable as a float 0..1; anything outside that range is clamped.
+type simResponse struct {
+	SameMarket bool    `json:"same_market"`
+	Confidence float64 `json:"confidence"`
+	Reason     string  `json:"reason"`
+}
+
+// Classify sends the paired metadata to Mistral and parses the JSON response.
+// No web scraping — the classifier works from name/city/year alone, which is
+// enough for the common cases (same venue listed on two different calendars,
+// editing typos, cross-year recurrence).
+func (m *MistralSimilarityClassifier) Classify(ctx context.Context, a, b SimilarityRow) (Verdict, error) {
+	if m.Client == nil {
+		return Verdict{}, errors.New("mistral similarity classifier not configured")
+	}
+	systemPrompt := simSystemPrompt()
+	userPrompt := simUserPrompt(a, b)
+
+	result, err := m.Client.Pass2(ctx, systemPrompt, userPrompt)
+	if err != nil {
+		return Verdict{}, fmt.Errorf("pass2: %w", err)
+	}
+
+	var parsed simResponse
+	if err := json.Unmarshal([]byte(result.Content), &parsed); err != nil {
+		return Verdict{}, fmt.Errorf("parse response: %w (content=%q)", err, result.Content)
+	}
+
+	// Clamp confidence to [0,1]; the model occasionally returns 1.2 or -0.1.
+	conf := parsed.Confidence
+	if conf < 0 {
+		conf = 0
+	}
+	if conf > 1 {
+		conf = 1
+	}
+
+	return Verdict{
+		Same:         parsed.SameMarket,
+		Confidence:   conf,
+		Reason:       strings.TrimSpace(parsed.Reason),
+		Model:        result.Model,
+		ClassifiedAt: time.Now().UTC(),
+	}, nil
+}
+
+func simSystemPrompt() string {
+	return strings.TrimSpace(`
+You decide whether two candidate entries refer to the same medieval market
+(Mittelaltermarkt) in the DACH region. Input: two objects each with a name,
+city, and year. Output a single JSON object:
+
+  {
+    "same_market": true|false,
+    "confidence": 0.0-1.0,     // how sure you are
+    "reason":     "..."         // short German justification (<= 140 chars)
+  }
+
+Rules:
+- Return ONLY the JSON object. No prose, no code fences.
+- "Same market" means same recurring event — same venue, same organiser,
+  same audience. A market and its anniversary edition in a later year ARE
+  the same market (just different editions).
+- Different cities = different markets, even if the name matches.
+- Rephrasings, typos, and umlaut differences (Dresden vs Straßburg vs
+  Strassburg) are the same market if the underlying identifiers align.
+- If the evidence is weak, return same_market=false with low confidence
+  rather than guessing. Low confidence is more useful than a wrong guess.
+`)
+}
+
+func simUserPrompt(a, b SimilarityRow) string {
+	// Keep the JSON compact; the model handles inline JSON better than
+	// pretty-printed when the task is "read two records".
+	ja, _ := json.Marshal(map[string]any{
+		"name": a.Name,
+		"city": a.Stadt,
+		"year": a.Year,
+	})
+	jb, _ := json.Marshal(map[string]any{
+		"name": b.Name,
+		"city": b.Stadt,
+		"year": b.Year,
+	})
+	return fmt.Sprintf("A: %s\nB: %s", ja, jb)
+}

backend/internal/domain/discovery/enrich/similarity_test.go

@@ -0,0 +1,118 @@
+package enrich
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+
+	"marktvogt.de/backend/internal/pkg/ai"
+)
+
+func TestSimilarityPairKey_Symmetric(t *testing.T) {
+	a := SimilarityRow{NameNormalized: "ritterfest dresden", Stadt: "Dresden", Year: 2026}
+	b := SimilarityRow{NameNormalized: "ritterfest leipzig", Stadt: "Leipzig", Year: 2026}
+
+	if SimilarityPairKey(a, b) != SimilarityPairKey(b, a) {
+		t.Error("pair key must be symmetric: (a,b) and (b,a) should produce identical keys")
+	}
+}
+
+func TestSimilarityPairKey_DifferentInputsDifferentKeys(t *testing.T) {
+	a := SimilarityRow{NameNormalized: "ritterfest dresden", Stadt: "Dresden", Year: 2026}
+	b := SimilarityRow{NameNormalized: "ritterfest leipzig", Stadt: "Leipzig", Year: 2026}
+	c := SimilarityRow{NameNormalized: "ritterfest dresden", Stadt: "Dresden", Year: 2027}
+
+	if SimilarityPairKey(a, b) == SimilarityPairKey(a, c) {
+		t.Error("different pairs must produce different keys")
+	}
+	// Stadt casing must not change the key.
+	d := SimilarityRow{NameNormalized: "ritterfest dresden", Stadt: "DRESDEN", Year: 2026}
+	if SimilarityPairKey(a, b) != SimilarityPairKey(d, b) {
+		t.Error("stadt casing drift must not change the key")
+	}
+}
+
+func TestMistralSimilarity_HappyPath(t *testing.T) {
+	client := &stubPass2{
+		result: ai.PassResult{
+			Content: `{"same_market":true,"confidence":0.82,"reason":"Gleicher Name, gleiche Stadt, gleiches Jahr."}`,
+			Model:   "mistral-large-latest",
+		},
+	}
+	c := NewMistralSimilarityClassifier(client)
+
+	got, err := c.Classify(context.Background(),
+		SimilarityRow{Name: "Mittelaltermarkt Dresden", Stadt: "Dresden", Year: 2026},
+		SimilarityRow{Name: "Mittelaltermarkt Dresden 2026", Stadt: "Dresden", Year: 2026},
+	)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if !got.Same {
+		t.Errorf("same = false; want true")
+	}
+	if got.Confidence < 0.8 || got.Confidence > 0.85 {
+		t.Errorf("confidence = %v; want ~0.82", got.Confidence)
+	}
+	if got.Reason == "" {
+		t.Error("reason missing")
+	}
+	if got.Model != "mistral-large-latest" {
+		t.Errorf("model = %q", got.Model)
+	}
+	// Prompt must carry both rows' identifying fields for the LLM to reason on.
+	if !strings.Contains(client.lastUser, "Mittelaltermarkt Dresden") {
+		t.Error("user prompt missing A.name")
+	}
+	if !strings.Contains(client.lastSystem, "same_market") {
+		t.Error("system prompt should describe the JSON schema (same_market key)")
+	}
+}
+
+func TestMistralSimilarity_ClampsConfidence(t *testing.T) {
+	tests := []struct {
+		name     string
+		raw      string
+		wantConf float64
+	}{
+		{"above 1 clamps to 1", `{"same_market":true,"confidence":1.4,"reason":"x"}`, 1.0},
+		{"below 0 clamps to 0", `{"same_market":false,"confidence":-0.3,"reason":"x"}`, 0.0},
+		{"in range passes through", `{"same_market":true,"confidence":0.5,"reason":"x"}`, 0.5},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			c := NewMistralSimilarityClassifier(&stubPass2{result: ai.PassResult{Content: tc.raw}})
+			got, err := c.Classify(context.Background(), SimilarityRow{}, SimilarityRow{})
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if got.Confidence != tc.wantConf {
+				t.Errorf("confidence = %v; want %v", got.Confidence, tc.wantConf)
+			}
+		})
+	}
+}
+
+func TestMistralSimilarity_PropagatesPass2Error(t *testing.T) {
+	c := NewMistralSimilarityClassifier(&stubPass2{err: errors.New("mistral down")})
+	_, err := c.Classify(context.Background(), SimilarityRow{}, SimilarityRow{})
+	if err == nil {
+		t.Fatal("expected error; got nil")
+	}
+}
+
+func TestMistralSimilarity_RejectsBadJSON(t *testing.T) {
+	c := NewMistralSimilarityClassifier(&stubPass2{result: ai.PassResult{Content: "not json at all"}})
+	_, err := c.Classify(context.Background(), SimilarityRow{}, SimilarityRow{})
+	if err == nil {
+		t.Fatal("expected parse error; got nil")
+	}
+}
+
+func TestNoopSimilarityClassifier_Errors(t *testing.T) {
+	_, err := NoopSimilarityClassifier{}.Classify(context.Background(), SimilarityRow{}, SimilarityRow{})
+	if err == nil {
+		t.Error("NoopSimilarityClassifier should return error — it's the fallback when AI is disabled")
+	}
+}

backend/internal/domain/discovery/handler.go

@@ -386,6 +386,36 @@ func (h *Handler) EnrichLLM(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"data": payload})
 }
 
+// ClassifySimilarPair runs the LLM duplicate-tiebreaker on the two queue
+// rows identified by URL params :aid and :bid. Synchronous, 15s deadline —
+// the call is short (no scraping) so the operator can click and immediately
+// see the verdict.
+func (h *Handler) ClassifySimilarPair(c *gin.Context) {
+	aID, err := uuid.Parse(c.Param("aid"))
+	if err != nil {
+		apiErr := apierror.BadRequest("invalid_id", "invalid queue id A")
+		c.JSON(apiErr.Status, apierror.NewResponse(apiErr))
+		return
+	}
+	bID, err := uuid.Parse(c.Param("bid"))
+	if err != nil {
+		apiErr := apierror.BadRequest("invalid_id", "invalid queue id B")
+		c.JSON(apiErr.Status, apierror.NewResponse(apiErr))
+		return
+	}
+	ctx, cancel := context.WithTimeout(c.Request.Context(), 15*time.Second)
+	defer cancel()
+
+	verdict, err := h.service.ClassifySimilarPair(ctx, aID, bID)
+	if err != nil {
+		slog.WarnContext(ctx, "classify similar failed", "a", aID, "b", bID, "error", err)
+		apiErr := apierror.Internal("classify failed: " + err.Error())
+		c.JSON(apiErr.Status, apierror.NewResponse(apiErr))
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"data": verdict})
+}
+
 func currentUserID(c *gin.Context) (uuid.UUID, bool) {
 	raw, exists := c.Get("user_id")
 	if !exists {

backend/internal/domain/discovery/handler_test.go

@@ -57,6 +57,7 @@ func TestCrawlHandlerReturns202AndStartsCrawl(t *testing.T) {
 		noopMarketCreator{},
 		nil,
 		nil,
+		nil,
 	)
 	h := NewHandler(svc, 0) // rate limit disabled
 
@@ -91,7 +92,7 @@ func TestCrawlHandlerReturns202AndStartsCrawl(t *testing.T) {
 
 // TestCrawlStatusInitialState verifies the zero state of a freshly constructed Handler.
 func TestCrawlStatusInitialState(t *testing.T) {
-	svc := NewService(newMockRepo(), &stubCrawlerRunner{}, noopLinkVerifier{}, noopMarketCreator{}, nil, nil)
+	svc := NewService(newMockRepo(), &stubCrawlerRunner{}, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, nil)
 	h := NewHandler(svc, 0)
 
 	w := httptest.NewRecorder()
@@ -141,7 +142,7 @@ func TestCrawlHandlerConcurrentReturnsTooManyRequests(t *testing.T) {
 		started: make(chan struct{}),
 		release: make(chan struct{}),
 	}
-	svc := NewService(newMockRepo(), bc, noopLinkVerifier{}, noopMarketCreator{}, nil, nil)
+	svc := NewService(newMockRepo(), bc, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, nil)
 	h := NewHandler(svc, 0) // rate limit disabled
 
 	// First request — returns 202 and spawns goroutine.
@@ -184,6 +185,7 @@ func TestCrawlHandlerRateLimit(t *testing.T) {
 		noopMarketCreator{},
 		nil,
 		nil,
+		nil,
 	)
 	// 1 per hour window.
 	h := NewHandler(svc, 1)

backend/internal/domain/discovery/mock_repo_test.go

@@ -36,6 +36,10 @@ type mockRepo struct {
 	getCacheFn          func(key string) (enrich.Enrichment, bool, error)
 	setCacheFn          func(key string, payload enrich.Enrichment, ttl time.Duration) error
 	listPendingEnrichFn func(limit int) ([]DiscoveredMarket, error)
+
+	// Similarity AI cache hooks.
+	getSimCacheFn func(pairKey string) (enrich.Verdict, bool, error)
+	setSimCacheFn func(pairKey string, v enrich.Verdict, ttl time.Duration) error
 }
 
 func (m *mockRepo) ListSeriesByCity(ctx context.Context, c string) ([]SeriesCandidate, error) {
@@ -122,6 +126,18 @@ func (m *mockRepo) ListPendingEnrichment(_ context.Context, limit int) ([]Discov
 	}
 	return nil, nil
 }
+func (m *mockRepo) GetSimilarityCache(_ context.Context, pairKey string) (enrich.Verdict, bool, error) {
+	if m.getSimCacheFn != nil {
+		return m.getSimCacheFn(pairKey)
+	}
+	return enrich.Verdict{}, false, nil
+}
+func (m *mockRepo) SetSimilarityCache(_ context.Context, pairKey string, v enrich.Verdict, ttl time.Duration) error {
+	if m.setSimCacheFn != nil {
+		return m.setSimCacheFn(pairKey, v, ttl)
+	}
+	return nil
+}
 
 // noopLinkVerifier passes every URL — used by tests to isolate from network.
 type noopLinkVerifier struct{}

backend/internal/domain/discovery/repository.go

@@ -40,6 +40,10 @@ type Repository interface {
 	// Cache operations keyed on sha256(name_normalized|stadt|year).
 	GetEnrichmentCache(ctx context.Context, key string) (enrich.Enrichment, bool, error)
 	SetEnrichmentCache(ctx context.Context, key string, payload enrich.Enrichment, ttl time.Duration) error
+
+	// Similarity AI cache — keyed on enrich.SimilarityPairKey.
+	GetSimilarityCache(ctx context.Context, pairKey string) (enrich.Verdict, bool, error)
+	SetSimilarityCache(ctx context.Context, pairKey string, v enrich.Verdict, ttl time.Duration) error
 }
 
 // SeriesCandidate is a minimal projection used for name-normalization comparison in Go.
@@ -426,3 +430,43 @@ ON CONFLICT (cache_key) DO UPDATE
 	}
 	return nil
 }
+
+// GetSimilarityCache returns a cached AI verdict for a (pairKey) or
+// (zero, false, nil) on miss/expiry. Expired entries are treated as misses.
+func (r *pgRepository) GetSimilarityCache(ctx context.Context, pairKey string) (enrich.Verdict, bool, error) {
+	var v enrich.Verdict
+	err := r.pool.QueryRow(ctx, `
+SELECT same, confidence, reason, model FROM similarity_ai_cache
+WHERE pair_key = $1
+  AND (expires_at IS NULL OR expires_at > now())`, pairKey).Scan(&v.Same, &v.Confidence, &v.Reason, &v.Model)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return enrich.Verdict{}, false, nil
+	}
+	if err != nil {
+		return enrich.Verdict{}, false, fmt.Errorf("similarity cache get: %w", err)
+	}
+	return v, true, nil
+}
+
+// SetSimilarityCache upserts a verdict. ttl=0 means "no expiry" (nullable).
+func (r *pgRepository) SetSimilarityCache(ctx context.Context, pairKey string, v enrich.Verdict, ttl time.Duration) error {
+	var expiresAt *time.Time
+	if ttl > 0 {
+		t := time.Now().Add(ttl)
+		expiresAt = &t
+	}
+	_, err := r.pool.Exec(ctx, `
+INSERT INTO similarity_ai_cache (pair_key, same, confidence, reason, model, expires_at)
+VALUES ($1, $2, $3, $4, $5, $6)
+ON CONFLICT (pair_key) DO UPDATE
+  SET same       = EXCLUDED.same,
+      confidence = EXCLUDED.confidence,
+      reason     = EXCLUDED.reason,
+      model      = EXCLUDED.model,
+      created_at = now(),
+      expires_at = EXCLUDED.expires_at`, pairKey, v.Same, v.Confidence, v.Reason, v.Model, expiresAt)
+	if err != nil {
+		return fmt.Errorf("similarity cache set: %w", err)
+	}
+	return nil
+}

backend/internal/domain/discovery/routes.go

@@ -26,6 +26,8 @@ func RegisterRoutes(
 		admin.GET("/queue/:id/similar", h.Similar)
 		// Per-row LLM enrichment (MR 3b). Synchronous — operator waits.
 		admin.POST("/queue/:id/enrich", h.EnrichLLM)
+		// Per-pair AI similarity tiebreak (MR 4). Synchronous; short call.
+		admin.POST("/queue/:aid/similar/:bid/classify", h.ClassifySimilarPair)
 		// Manual crawl trigger — subject to hourly rate limit.
 		admin.POST("/crawl-manual", h.Crawl)
 		// Async crawl status polling.

backend/internal/domain/discovery/service.go

@@ -57,13 +57,17 @@ type Service struct {
 	// llmEnricher is the AI-backed fallback pass. Nil-safe via NoopLLMEnricher
 	// in test wiring; production code passes a real MistralLLMEnricher.
 	llmEnricher enrich.LLMEnricher
+	// simClassifier is the AI-backed duplicate tiebreaker. Nil-safe via
+	// NoopSimilarityClassifier.
+	simClassifier enrich.SimilarityClassifier
 }
 
 // NewService constructs a Service wired for the crawler-driven Crawl path.
 // geocoder may be nil; in that case crawl-enrich runs consolidation only and
 // skips the lat/lng step. llm may be nil; per-row LLM enrichment then
-// returns an error instead of attempting a call.
-func NewService(repo Repository, cr crawlerRunner, lc linkVerifier, mc marketCreator, gc Geocoder, llm enrich.LLMEnricher) *Service {
+// returns an error instead of attempting a call. simClassifier may be nil;
+// ClassifySimilarPair returns an error rather than a zero-confidence answer.
+func NewService(repo Repository, cr crawlerRunner, lc linkVerifier, mc marketCreator, gc Geocoder, llm enrich.LLMEnricher, sim enrich.SimilarityClassifier) *Service {
 	return &Service{
 		repo:          repo,
 		crawler:       cr,
@@ -71,6 +75,7 @@ func NewService(repo Repository, cr crawlerRunner, lc linkVerifier, mc marketCre
 		linkChecker:   lc,
 		geocoder:      gc,
 		llmEnricher:   llm,
+		simClassifier: sim,
 	}
 }
 
@@ -595,6 +600,68 @@ func (s *Service) RunLLMEnrichOne(ctx context.Context, queueID uuid.UUID) (enric
 	return merged, nil
 }
 
+// ClassifySimilarPair runs the AI classifier on the two queue rows identified
+// by aID and bID, returning a verdict about whether they're the same market.
+// Cache-first: a content-keyed entry (enrich.SimilarityPairKey) shortcuts the
+// LLM call when the same pair has been classified before.
+//
+// Intended for operator-triggered "AI tiebreak" on ambiguous similarity
+// matches. The crawl-time auto-merge in MR 7 will call this on its own.
+func (s *Service) ClassifySimilarPair(ctx context.Context, aID, bID uuid.UUID) (enrich.Verdict, error) {
+	if s.simClassifier == nil {
+		return enrich.Verdict{}, errors.New("similarity classifier not configured")
+	}
+	if aID == bID {
+		return enrich.Verdict{}, errors.New("cannot classify a row against itself")
+	}
+	a, err := s.repo.GetDiscovered(ctx, aID)
+	if err != nil {
+		return enrich.Verdict{}, fmt.Errorf("load row A: %w", err)
+	}
+	b, err := s.repo.GetDiscovered(ctx, bID)
+	if err != nil {
+		return enrich.Verdict{}, fmt.Errorf("load row B: %w", err)
+	}
+
+	rowA := rowToSimilarity(a)
+	rowB := rowToSimilarity(b)
+	pairKey := enrich.SimilarityPairKey(rowA, rowB)
+
+	if cached, hit, err := s.repo.GetSimilarityCache(ctx, pairKey); err != nil {
+		slog.WarnContext(ctx, "similarity cache get failed; continuing",
+			"pair_key", pairKey, "error", err)
+	} else if hit {
+		return cached, nil
+	}
+
+	verdict, err := s.simClassifier.Classify(ctx, rowA, rowB)
+	if err != nil {
+		return enrich.Verdict{}, fmt.Errorf("classify: %w", err)
+	}
+
+	if err := s.repo.SetSimilarityCache(ctx, pairKey, verdict, enrich.DefaultSimilarityCacheTTL); err != nil {
+		slog.WarnContext(ctx, "similarity cache set failed; continuing",
+			"pair_key", pairKey, "error", err)
+	}
+	return verdict, nil
+}
+
+// rowToSimilarity adapts a DiscoveredMarket to the narrow SimilarityRow the
+// enrich package consumes. Year comes from StartDatum (0 when unknown).
+func rowToSimilarity(r DiscoveredMarket) enrich.SimilarityRow {
+	year := 0
+	if r.StartDatum != nil {
+		year = r.StartDatum.Year()
+	}
+	return enrich.SimilarityRow{
+		NameNormalized: r.NameNormalized,
+		Stadt:          r.Stadt,
+		Year:           year,
+		Name:           r.MarktName,
+		Quellen:        r.Quellen,
+	}
+}
+
 // CrawlEnrichSummary reports the outcome of one RunCrawlEnrichAll pass.
 // Mirrors CrawlSummary's shape so the admin UI can reuse its render path.
 type CrawlEnrichSummary struct {

backend/internal/domain/discovery/service_test.go

@@ -147,7 +147,7 @@ func TestAccept_NewSeries_CallsCreate(t *testing.T) {
 		markAcceptedFn: func(_ context.Context, _ pgx.Tx, _, _, _ uuid.UUID) error { return nil },
 	}
 	mc := &stubCreator{}
-	svc := NewService(m, nil, noopLinkVerifier{}, mc, nil, nil)
+	svc := NewService(m, nil, noopLinkVerifier{}, mc, nil, nil, nil)
 	_, _, err := svc.Accept(context.Background(), qID, uuid.New())
 	if err != nil {
 		t.Fatalf("accept err: %v", err)
@@ -169,7 +169,7 @@ func TestAccept_ExistingSeries_CallsCreateEditionForSeries(t *testing.T) {
 		markAcceptedFn: func(_ context.Context, _ pgx.Tx, _, _, _ uuid.UUID) error { return nil },
 	}
 	mc := &stubCreator{}
-	svc := NewService(m, nil, noopLinkVerifier{}, mc, nil, nil)
+	svc := NewService(m, nil, noopLinkVerifier{}, mc, nil, nil, nil)
 	_, _, err := svc.Accept(context.Background(), uuid.New(), uuid.New())
 	if err != nil {
 		t.Fatalf("accept err: %v", err)
@@ -200,7 +200,7 @@ func TestServiceCrawlHappyPath(t *testing.T) {
 			PerSourceMS: map[string]int64{"marktkalendarium": 1},
 		},
 	}
-	svc := NewService(repo, sc, lc, noopMarketCreator{}, nil, nil)
+	svc := NewService(repo, sc, lc, noopMarketCreator{}, nil, nil, nil)
 
 	summary, err := svc.Crawl(context.Background())
 	if err != nil {
@@ -241,7 +241,7 @@ func TestServiceCrawlDedupQueue(t *testing.T) {
 			},
 		},
 	}
-	svc := NewService(repo, sc, noopLinkVerifier{}, noopMarketCreator{}, nil, nil)
+	svc := NewService(repo, sc, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, nil)
 
 	summary, err := svc.Crawl(context.Background())
 	if err != nil {
@@ -272,7 +272,7 @@ func TestServiceCrawlDefaultsEndDate(t *testing.T) {
 			},
 		},
 	}
-	svc := NewService(repo, sc, noopLinkVerifier{}, noopMarketCreator{}, nil, nil)
+	svc := NewService(repo, sc, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, nil)
 
 	if _, err := svc.Crawl(context.Background()); err != nil {
 		t.Fatal(err)
@@ -318,7 +318,7 @@ func TestServiceCrawlDetachesInsertContextFromRequestCtx(t *testing.T) {
 			},
 		},
 	}
-	svc := NewService(repo, sc, ctxAwareLinkVerifier{}, noopMarketCreator{}, nil, nil)
+	svc := NewService(repo, sc, ctxAwareLinkVerifier{}, noopMarketCreator{}, nil, nil, nil)
 
 	// Cancel the context BEFORE Crawl runs — simulates gateway timeout
 	// that fires while the handler is still mid-run.
@@ -343,7 +343,7 @@ func TestListPendingQueuePaged_ReturnsBothRowsAndTotal(t *testing.T) {
 	m := &mockRepo{
 		countQueueFn: func(_ context.Context, _ string) (int, error) { return 42, nil },
 	}
-	svc := NewService(m, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, nil)
+	svc := NewService(m, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, nil)
 	rows, total, err := svc.ListPendingQueuePaged(context.Background(), 50, 0)
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
@@ -375,7 +375,7 @@ func TestServiceCrawlPersistsSourcesAndContributions(t *testing.T) {
 			},
 		},
 	}
-	svc := NewService(repo, sc, noopLinkVerifier{}, noopMarketCreator{}, nil, nil)
+	svc := NewService(repo, sc, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, nil)
 
 	if _, err := svc.Crawl(context.Background()); err != nil {
 		t.Fatal(err)
@@ -424,7 +424,7 @@ func TestServiceCrawlMultiSourceHighKonfidenz(t *testing.T) {
 			},
 		},
 	}
-	svc := NewService(repo, sc, noopLinkVerifier{}, noopMarketCreator{}, nil, nil)
+	svc := NewService(repo, sc, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, nil)
 
 	summary, err := svc.Crawl(context.Background())
 	if err != nil {
@@ -486,7 +486,7 @@ func TestRunCrawlEnrichAll_HappyPath(t *testing.T) {
 		},
 	}
 	gc := stubGeocoder{lat: ptrFloat(51.05), lng: ptrFloat(13.74)}
-	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, gc, nil)
+	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, gc, nil, nil)
 
 	summary, err := svc.RunCrawlEnrichAll(context.Background())
 	if err != nil {
@@ -547,7 +547,7 @@ func TestRunCrawlEnrichAll_SetEnrichmentFailure(t *testing.T) {
 			return nil
 		},
 	}
-	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, nil)
+	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, nil)
 
 	summary, err := svc.RunCrawlEnrichAll(context.Background())
 	if err != nil {
@@ -639,7 +639,7 @@ func TestRunLLMEnrichOne_HappyPath(t *testing.T) {
 		},
 	}
 	llm := &stubLLMEnricher{result: llmResult}
-	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, llm)
+	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, llm, nil)
 
 	got, err := svc.RunLLMEnrichOne(context.Background(), rowID)
 	if err != nil {
@@ -709,7 +709,7 @@ func TestRunLLMEnrichOne_CacheHitSkipsLLM(t *testing.T) {
 		},
 	}
 	llm := &stubLLMEnricher{}
-	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, llm)
+	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, llm, nil)
 
 	got, err := svc.RunLLMEnrichOne(context.Background(), rowID)
 	if err != nil {
@@ -738,7 +738,7 @@ func TestRunLLMEnrichOne_LLMErrorMarksFailed(t *testing.T) {
 		},
 	}
 	llm := &stubLLMEnricher{err: errors.New("mistral down")}
-	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, llm)
+	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, llm, nil)
 
 	_, err := svc.RunLLMEnrichOne(context.Background(), rowID)
 	if err == nil {
@@ -761,7 +761,7 @@ func TestRunCrawlEnrichAll_EmptyQueueNoOp(t *testing.T) {
 			return nil
 		},
 	}
-	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, nil)
+	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, nil)
 
 	summary, err := svc.RunCrawlEnrichAll(context.Background())
 	if err != nil {
@@ -774,3 +774,129 @@ func TestRunCrawlEnrichAll_EmptyQueueNoOp(t *testing.T) {
 		t.Errorf("SetEnrichment called %d times on empty queue", writes)
 	}
 }
+
+// stubSimilarityClassifier returns canned verdicts or errors.
+type stubSimilarityClassifier struct {
+	result enrich.Verdict
+	err    error
+	calls  int
+}
+
+func (s *stubSimilarityClassifier) Classify(_ context.Context, _, _ enrich.SimilarityRow) (enrich.Verdict, error) {
+	s.calls++
+	return s.result, s.err
+}
+
+// TestClassifySimilarPair_HappyPath: cache miss → LLM → cache write → return.
+func TestClassifySimilarPair_HappyPath(t *testing.T) {
+	aID, bID := uuid.New(), uuid.New()
+	start := time.Date(2026, 5, 1, 0, 0, 0, 0, time.UTC)
+	rowA := DiscoveredMarket{ID: aID, MarktName: "Ritterfest Dresden", Stadt: "Dresden", NameNormalized: "ritterfest dresden", StartDatum: &start}
+	rowB := DiscoveredMarket{ID: bID, MarktName: "Mittelaltermarkt Dresden 2026", Stadt: "Dresden", NameNormalized: "mittelaltermarkt dresden", StartDatum: &start}
+
+	var cacheSet struct {
+		called  bool
+		verdict enrich.Verdict
+	}
+	repo := &mockRepo{
+		getDiscoveredFn: func(_ context.Context, id uuid.UUID) (DiscoveredMarket, error) {
+			if id == aID {
+				return rowA, nil
+			}
+			return rowB, nil
+		},
+		setSimCacheFn: func(_ string, v enrich.Verdict, _ time.Duration) error {
+			cacheSet.called = true
+			cacheSet.verdict = v
+			return nil
+		},
+	}
+	sim := &stubSimilarityClassifier{
+		result: enrich.Verdict{Same: false, Confidence: 0.6, Reason: "Unterschiedliche Namen."},
+	}
+	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, sim)
+
+	got, err := svc.ClassifySimilarPair(context.Background(), aID, bID)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if sim.calls != 1 {
+		t.Errorf("classifier called %d times; want 1", sim.calls)
+	}
+	if got.Same || got.Confidence != 0.6 {
+		t.Errorf("verdict = %+v; want same=false, confidence=0.6", got)
+	}
+	if !cacheSet.called {
+		t.Error("expected SetSimilarityCache to be called on a cache miss")
+	}
+	if cacheSet.verdict.Confidence != 0.6 {
+		t.Errorf("cached verdict lost confidence: %+v", cacheSet.verdict)
+	}
+}
+
+// TestClassifySimilarPair_CacheHitSkipsLLM: cache hit returns directly,
+// no classifier call.
+func TestClassifySimilarPair_CacheHitSkipsLLM(t *testing.T) {
+	aID, bID := uuid.New(), uuid.New()
+	cached := enrich.Verdict{Same: true, Confidence: 0.9, Reason: "same venue"}
+
+	repo := &mockRepo{
+		getDiscoveredFn: func(context.Context, uuid.UUID) (DiscoveredMarket, error) {
+			return DiscoveredMarket{}, nil
+		},
+		getSimCacheFn: func(string) (enrich.Verdict, bool, error) {
+			return cached, true, nil
+		},
+	}
+	sim := &stubSimilarityClassifier{}
+	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, sim)
+
+	got, err := svc.ClassifySimilarPair(context.Background(), aID, bID)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if sim.calls != 0 {
+		t.Errorf("classifier called %d times on cache hit; want 0", sim.calls)
+	}
+	if !got.Same || got.Confidence != 0.9 {
+		t.Errorf("cached verdict not returned: %+v", got)
+	}
+}
+
+// TestClassifySimilarPair_RejectsSelfComparison: the pair-key scheme would
+// collapse (a,a) to a single key which never tells you anything useful.
+func TestClassifySimilarPair_RejectsSelfComparison(t *testing.T) {
+	id := uuid.New()
+	svc := NewService(&mockRepo{}, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, &stubSimilarityClassifier{})
+
+	_, err := svc.ClassifySimilarPair(context.Background(), id, id)
+	if err == nil {
+		t.Error("expected error on self-comparison; got nil")
+	}
+}
+
+// TestClassifySimilarPair_LLMErrorPropagates: classifier errors surface;
+// cache is not written.
+func TestClassifySimilarPair_LLMErrorPropagates(t *testing.T) {
+	aID, bID := uuid.New(), uuid.New()
+	cacheWritten := false
+	repo := &mockRepo{
+		getDiscoveredFn: func(context.Context, uuid.UUID) (DiscoveredMarket, error) {
+			return DiscoveredMarket{}, nil
+		},
+		setSimCacheFn: func(string, enrich.Verdict, time.Duration) error {
+			cacheWritten = true
+			return nil
+		},
+	}
+	sim := &stubSimilarityClassifier{err: errors.New("mistral 500")}
+	svc := NewService(repo, nil, noopLinkVerifier{}, noopMarketCreator{}, nil, nil, sim)
+
+	_, err := svc.ClassifySimilarPair(context.Background(), aID, bID)
+	if err == nil {
+		t.Fatal("expected error; got nil")
+	}
+	if cacheWritten {
+		t.Error("cache should not be written when classifier errors")
+	}
+}

backend/internal/server/routes.go

@@ -79,11 +79,13 @@ func (s *Server) registerRoutes() {
 	// Per-row LLM enrichment (MR 3b). Operator-triggered only; disabled rows
 	// fall through via NoopLLMEnricher when AI isn't configured.
 	var llmEnricher enrich.LLMEnricher = enrich.NoopLLMEnricher{}
+	var simClassifier enrich.SimilarityClassifier = enrich.NoopSimilarityClassifier{}
 	if aiClient.Enabled() {
 		scraper := scrape.New(s.cfg.Discovery.CrawlerUserAgent)
 		llmEnricher = enrich.NewMistralLLMEnricher(aiClient, scraper)
+		simClassifier = enrich.NewMistralSimilarityClassifier(aiClient)
 	}
-	discoveryService := discovery.NewService(discoveryRepo, crawlerInstance, discovery.NewLinkChecker(), marketSvc, geocoder, llmEnricher)
+	discoveryService := discovery.NewService(discoveryRepo, crawlerInstance, discovery.NewLinkChecker(), marketSvc, geocoder, llmEnricher, simClassifier)
 	discoveryHandler := discovery.NewHandler(discoveryService, s.cfg.Discovery.CrawlerManualRateLimitPerHour)
 	requireTickToken := middleware.RequireBearerToken(s.cfg.Discovery.Token)
 	discovery.RegisterRoutes(v1, discoveryHandler, requireAuth, requireAdmin, requireTickToken)

backend/migrations/000020_similarity_ai_cache.down.sql

@@ -0,0 +1,2 @@
+DROP INDEX IF EXISTS idx_similarity_ai_cache_expires_at;
+DROP TABLE IF EXISTS similarity_ai_cache;

backend/migrations/000020_similarity_ai_cache.up.sql

@@ -0,0 +1,18 @@
+-- Cache for LLM-based similarity verdicts. Pair key is a sha256 over the
+-- content tuple (normalized_name|stadt|year for both rows, sorted) — NOT
+-- over queue UUIDs, so the cache survives queue row deletion (accept/reject)
+-- while still invalidating correctly when a row's identifying fields change.
+CREATE TABLE similarity_ai_cache (
+  pair_key   text        PRIMARY KEY,
+  same       bool        NOT NULL,
+  confidence real        NOT NULL,
+  reason     text        NOT NULL DEFAULT '',
+  model      text        NOT NULL DEFAULT '',
+  created_at timestamptz NOT NULL DEFAULT now(),
+  expires_at timestamptz
+);
+
+-- Opportunistic pruning; workers or manual jobs SELECT WHERE expires_at < now().
+CREATE INDEX idx_similarity_ai_cache_expires_at
+  ON similarity_ai_cache (expires_at)
+  WHERE expires_at IS NOT NULL;

web/src/routes/admin/discovery/+page.svelte

@@ -99,6 +99,7 @@
 		similarOpenId = id;
 		similarLoading = true;
 		similarEntries = [];
+		similarVerdicts = {};
 		try {
 			const res = await fetch(`/admin/discovery/queue/${id}/similar`);
 			if (!res.ok) {
@@ -114,6 +115,53 @@
 		}
 	}
 
+	// Per-pair AI similarity verdict. Keyed on the candidate's queue id since
+	// the "anchor" row (similarOpenId) is already known from context.
+	type SimilarityVerdict = {
+		same: boolean;
+		confidence: number;
+		reason: string;
+		model?: string;
+		classified_at?: string;
+	};
+	let similarVerdicts = $state<Record<string, SimilarityVerdict>>({});
+	let similarClassifying = $state(new Set<string>());
+
+	async function classifySimilar(anchorId: string, candidateId: string) {
+		if (similarClassifying.has(candidateId)) return;
+		const next = new Set(similarClassifying);
+		next.add(candidateId);
+		similarClassifying = next;
+		try {
+			const res = await fetch(
+				`/admin/discovery/queue/${anchorId}/similar/${candidateId}/classify`,
+				{ method: 'POST' }
+			);
+			if (!res.ok) {
+				similarVerdicts = {
+					...similarVerdicts,
+					[candidateId]: { same: false, confidence: 0, reason: `HTTP ${res.status}` }
+				};
+				return;
+			}
+			const body = await res.json();
+			similarVerdicts = { ...similarVerdicts, [candidateId]: body.data };
+		} catch (err) {
+			similarVerdicts = {
+				...similarVerdicts,
+				[candidateId]: {
+					same: false,
+					confidence: 0,
+					reason: err instanceof Error ? err.message : 'Fehler'
+				}
+			};
+		} finally {
+			const afterNext = new Set(similarClassifying);
+			afterNext.delete(candidateId);
+			similarClassifying = afterNext;
+		}
+	}
+
 	// Pagination helpers.
 	const totalPages = $derived(Math.ceil((data.total ?? 0) / data.limit));
 
@@ -777,7 +825,8 @@
 													<th class="pr-4 pb-1 font-medium">Markt</th>
 													<th class="pr-4 pb-1 font-medium">Stadt</th>
 													<th class="pr-4 pb-1 font-medium">Datum</th>
-													<th class="pb-1 font-medium">Konfidenz</th>
+													<th class="pr-4 pb-1 font-medium">Konfidenz</th>
+													<th class="pb-1 font-medium">AI-Verdict</th>
 												</tr>
 											</thead>
 											<tbody>
@@ -791,7 +840,7 @@
 														<td class="py-1 pr-4 whitespace-nowrap"
 															>{fmtDate(m.entry.start_datum)}</td
 														>
-														<td class="py-1">
+														<td class="py-1 pr-4">
 															<span
 																class="inline-block rounded px-1.5 py-0.5 {konfidenzClass(
 																	m.entry.konfidenz
@@ -800,6 +849,30 @@
 																{m.entry.konfidenz || '—'}
 															</span>
 														</td>
+														<td class="py-1">
+															{#if similarVerdicts[m.entry.id]}
+																{@const v = similarVerdicts[m.entry.id]}
+																<span
+																	class="inline-block rounded px-1.5 py-0.5 text-[10px] {v.same
+																		? 'bg-emerald-100 text-emerald-700 dark:bg-emerald-900/50 dark:text-emerald-300'
+																		: 'bg-stone-200 text-stone-700 dark:bg-stone-700 dark:text-stone-200'}"
+																	title={v.reason}
+																>
+																	{v.same ? '✓ same' : '✗ diff'}
+																	{(v.confidence * 100).toFixed(0)}%
+																</span>
+															{:else}
+																<button
+																	type="button"
+																	disabled={similarClassifying.has(m.entry.id)}
+																	onclick={() => classifySimilar(row.id, m.entry.id)}
+																	class="rounded bg-purple-100 px-1.5 py-0.5 text-[10px] text-purple-700 hover:bg-purple-200 disabled:cursor-not-allowed disabled:opacity-60 dark:bg-purple-900/50 dark:text-purple-300 dark:hover:bg-purple-900"
+																	title="LLM-Tiebreak: sind das derselbe Markt?"
+																>
+																	{similarClassifying.has(m.entry.id) ? 'AI…' : 'AI?'}
+																</button>
+															{/if}
+														</td>
 													</tr>
 												{/each}
 											</tbody>