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/backend/handlers.go

@@ -569,6 +569,30 @@ func (s *Server) handleReindex(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, 0, map[string]any{"slug": slug, "indexed": count})
 }
 
+// handleDedupChapters handles POST /api/admin/dedup-chapters/{slug}.
+// Removes duplicate chapters_idx records for a book, keeping the latest record
+// per chapter number. Returns the number of duplicate records deleted.
+func (s *Server) handleDedupChapters(w http.ResponseWriter, r *http.Request) {
+	slug := r.PathValue("slug")
+	if slug == "" {
+		jsonError(w, http.StatusBadRequest, "missing slug")
+		return
+	}
+
+	deleted, err := s.deps.BookWriter.DeduplicateChapters(r.Context(), slug)
+	if err != nil {
+		s.deps.Log.Error("dedup-chapters failed", "slug", slug, "err", err)
+		writeJSON(w, http.StatusInternalServerError, map[string]any{
+			"error":   err.Error(),
+			"deleted": deleted,
+		})
+		return
+	}
+
+	s.deps.Log.Info("dedup-chapters complete", "slug", slug, "deleted", deleted)
+	writeJSON(w, 0, map[string]any{"slug": slug, "deleted": deleted})
+}
+
 // ── Audio ──────────────────────────────────────────────────────────────────────
 
 // handleAudioGenerate handles POST /api/audio/{slug}/{n}.

backend/internal/backend/server.go

@@ -204,6 +204,9 @@ func (s *Server) ListenAndServe(ctx context.Context) error {
 	mux.HandleFunc("POST /api/admin/text-gen/description", s.handleAdminTextGenDescription)
 	mux.HandleFunc("POST /api/admin/text-gen/description/apply", s.handleAdminTextGenApplyDescription)
 
+	// Admin data repair endpoints
+	mux.HandleFunc("POST /api/admin/dedup-chapters/{slug}", s.handleDedupChapters)
+
 	// Voices list
 	mux.HandleFunc("GET /api/voices", s.handleVoices)