fix: prevent duplicate chapters_idx records + add dedup endpoint

- Fix upsertChapterIdx race: use conflict-retry pattern (mirrors WriteMetadata)
  so concurrent goroutines don't double-POST the same chapter number
- Add DeduplicateChapters to BookWriter interface and Store implementation;
  keeps the latest record per (slug, number) and deletes extras
- Wire POST /api/admin/dedup-chapters/{slug} handler in server.go

backend/internal/storage/store.go

@@ -130,7 +130,16 @@ func (s *Store) upsertChapterIdx(ctx context.Context, slug string, ref domain.Ch
 		return err
 	}
 	if len(items) == 0 {
-		return s.pb.post(ctx, "/api/collections/chapters_idx/records", payload, nil)
+		postErr := s.pb.post(ctx, "/api/collections/chapters_idx/records", payload, nil)
+		if postErr == nil {
+			return nil
+		}
+		// POST failed — a concurrent writer may have inserted the same slug+number.
+		// Re-fetch and fall through to PATCH (mirrors WriteMetadata retry pattern).
+		items, err = s.pb.listAll(ctx, "chapters_idx", filter, "")
+		if err != nil || len(items) == 0 {
+			return postErr // original POST error is more informative
+		}
 	}
 	var rec struct {
 		ID string `json:"id"`
@@ -139,6 +148,59 @@ func (s *Store) upsertChapterIdx(ctx context.Context, slug string, ref domain.Ch
 	return s.pb.patch(ctx, fmt.Sprintf("/api/collections/chapters_idx/records/%s", rec.ID), payload)
 }
 
+// DeduplicateChapters removes duplicate chapters_idx records for slug.
+// For each chapter number that has more than one record, it keeps the record
+// with the latest "updated" timestamp and deletes the rest.
+// Returns the number of records deleted.
+func (s *Store) DeduplicateChapters(ctx context.Context, slug string) (int, error) {
+	filter := fmt.Sprintf(`slug=%q`, slug)
+	items, err := s.pb.listAll(ctx, "chapters_idx", filter, "number")
+	if err != nil {
+		return 0, fmt.Errorf("DeduplicateChapters: list: %w", err)
+	}
+
+	type record struct {
+		ID      string `json:"id"`
+		Number  int    `json:"number"`
+		Updated string `json:"updated"`
+	}
+
+	// Group records by chapter number.
+	byNumber := make(map[int][]record)
+	for _, raw := range items {
+		var rec record
+		if err := json.Unmarshal(raw, &rec); err != nil || rec.ID == "" {
+			continue
+		}
+		byNumber[rec.Number] = append(byNumber[rec.Number], rec)
+	}
+
+	deleted := 0
+	for _, recs := range byNumber {
+		if len(recs) <= 1 {
+			continue
+		}
+		// Keep the record with the latest Updated timestamp; delete the rest.
+		keep := 0
+		for i := 1; i < len(recs); i++ {
+			if recs[i].Updated > recs[keep].Updated {
+				keep = i
+			}
+		}
+		for i, rec := range recs {
+			if i == keep {
+				continue
+			}
+			if delErr := s.pb.delete(ctx, fmt.Sprintf("/api/collections/chapters_idx/records/%s", rec.ID)); delErr != nil {
+				s.log.Warn("DeduplicateChapters: delete failed", "slug", slug, "number", rec.Number, "id", rec.ID, "err", delErr)
+				continue
+			}
+			deleted++
+		}
+	}
+	return deleted, nil
+}
+
 // ── BookReader ────────────────────────────────────────────────────────────────
 
 type pbBook struct {