fix(discovery): sort source_contributions by rank on read

MergePendingSources re-aggregates the jsonb array with
ORDER BY source_name for DB determinism, but the admin UI treats
index 0 as "Rang 1 = winning source." Legacy auto-merged rows were
therefore surfacing mittelalterkalender (alphabetically first) as
Rang 1 instead of the actual rank-1 source mittelaltermarkt_online.

- Export crawler.SourceRank (was unexported rankOf) so other packages
  in the discovery domain can reference the canonical rank map.
- scanDiscoveredMarket: sort.SliceStable SourceContributions by rank
  after unmarshal. Every read path now sees contributions in rank
  order regardless of how they were persisted; legacy rows
  self-correct on next read, no migration needed.

backend/internal/domain/discovery/crawler/merger.go

@@ -95,7 +95,7 @@ func mergeKey(r RawEvent) string {
 func mergeGroup(raws []RawEvent) MergedEvent {
 	// Sort by source rank so "first non-empty" == "best source's value".
 	sort.SliceStable(raws, func(i, j int) bool {
-		return rankOf(raws[i].SourceName) < rankOf(raws[j].SourceName)
+		return SourceRank(raws[i].SourceName) < SourceRank(raws[j].SourceName)
 	})
 
 	m := MergedEvent{}
@@ -176,7 +176,7 @@ func mergeGroup(raws []RawEvent) MergedEvent {
 	return m
 }
 
-func rankOf(name string) int {
+func SourceRank(name string) int {
 	if r, ok := sourceRank[name]; ok {
 		return r
 	}

backend/internal/domain/discovery/repository.go

@@ -6,12 +6,14 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"sort"
 	"time"
 
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
 
+	"marktvogt.de/backend/internal/domain/discovery/crawler"
 	"marktvogt.de/backend/internal/domain/discovery/enrich"
 )
 
@@ -350,6 +352,14 @@ func scanDiscoveredMarket(s scanner) (DiscoveredMarket, error) {
 	if d.SourceContributions == nil {
 		d.SourceContributions = []SourceContribution{}
 	}
+	// Persisted order (alphabetical from MergePendingSources) doesn't match the
+	// rank order the UI relies on for "Rang 1 = winning source". Sort by rank
+	// here so every read path sees contributions in rank order regardless of
+	// how they were stored.
+	sort.SliceStable(d.SourceContributions, func(i, j int) bool {
+		return crawler.SourceRank(d.SourceContributions[i].SourceName) <
+			crawler.SourceRank(d.SourceContributions[j].SourceName)
+	})
 	return d, nil
 }