feat(ai): add PromptHash to ProviderError + log on schema violation

promptHashShort(system+"\x00"+user)[:12] computed on ErrSchemaViolation
and attached to ProviderError.PromptHash. research.go schema-violation
log now includes prompt_hash for cross-referencing ai_usage rows.

backend/internal/domain/market/research.go

@@ -80,7 +80,11 @@ func (h *ResearchHandler) Research(c *gin.Context) {
 				c.JSON(http.StatusServiceUnavailable, apierror.NewResponse(apierror.BadRequest("rate_limited", "KI rate limit erreicht, bitte kurz warten")))
 				return
 			case ai.ErrSchemaViolation:
-				slog.ErrorContext(ctx, "research schema violation", "market_id", id, "raw", pe.RawOutput, "inner", pe.Inner)
+				slog.ErrorContext(ctx, "research schema violation",
+					"market_id", id,
+					"prompt_hash", pe.PromptHash,
+					"raw", pe.RawOutput,
+					"inner", pe.Inner)
 				c.JSON(http.StatusInternalServerError, apierror.NewResponse(apierror.Internal("Modell hat ungültige Ausgabe geliefert")))
 				return
 			case ai.ErrInvalidRequest:

backend/internal/pkg/ai/errors.go

@@ -42,11 +42,12 @@ func (c ErrorCode) String() string {
 }
 
 type ProviderError struct {
-	Code      ErrorCode
-	Message   string
-	Retryable bool
-	Inner     error
-	RawOutput string
+	Code       ErrorCode
+	Message    string
+	Retryable  bool
+	Inner      error
+	RawOutput  string
+	PromptHash string // sha256(system+"\x00"+user)[:12], set on ErrSchemaViolation
 }
 
 func (e *ProviderError) Error() string {

backend/internal/pkg/ai/gemini.go

@@ -2,6 +2,8 @@ package ai
 
 import (
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"fmt"
 	"sort"
@@ -258,12 +260,14 @@ func (p *GeminiProvider) Chat(ctx context.Context, req *ChatRequest) (*ChatRespo
 	text := resp.Text()
 	if len(req.JSONSchema) > 0 {
 		if verr := ValidateSchema(req.JSONSchema, []byte(text)); verr != nil {
+			promptHash := promptHashShort(req.SystemPrompt, req.UserMessage)
 			return nil, &ProviderError{
-				Code:      ErrSchemaViolation,
-				Message:   fmt.Sprintf("response does not match schema: %v", verr),
-				Retryable: true,
-				Inner:     verr,
-				RawOutput: text,
+				Code:       ErrSchemaViolation,
+				Message:    fmt.Sprintf("response does not match schema: %v", verr),
+				Retryable:  true,
+				Inner:      verr,
+				RawOutput:  text,
+				PromptHash: promptHash,
 			}
 		}
 	}
@@ -330,6 +334,13 @@ func (p *GeminiProvider) record(ctx context.Context, e UsageEvent) {
 	_ = p.recorder.Record(ctx, e)
 }
 
+// promptHashShort returns the first 12 hex chars of sha256(system+"\x00"+user).
+// Used to correlate schema-violation errors with the prompt that produced them.
+func promptHashShort(system, user string) string {
+	h := sha256.Sum256([]byte(system + "\x00" + user))
+	return hex.EncodeToString(h[:])[:12]
+}
+
 // schemaFromMap converts a raw JSON-schema map to genai.Schema for structured output.
 func schemaFromMap(m map[string]any) *genai.Schema {
 	s := &genai.Schema{}