fix: repair notification system (broken type assertion + wrong PocketBase filter quotes)

- Add bookstore.NotificationStore interface and wire it directly to *storage.Store
  in Dependencies, bypassing the Asynq wrapper that caused Producer.(*storage.Store)
  to fail with a 500 on every notification endpoint when Redis is configured
- Replace s.deps.Producer.(*storage.Store) type assertion in all 5 notification
  handlers with s.deps.NotificationStore (nil-safe, always works)
- Fix PocketBase filter single-quote bug in ListNotifications, ClearAllNotifications,
  and MarkAllNotificationsRead (PocketBase requires double quotes for string values)

backend/internal/storage/store.go

@@ -773,7 +773,7 @@ func (s *Store) CreateNotification(ctx context.Context, userID, title, message,
 
 // ListNotifications returns notifications for a user.
 func (s *Store) ListNotifications(ctx context.Context, userID string, limit int) ([]map[string]any, error) {
-	filter := fmt.Sprintf("user_id='%s'", userID)
+	filter := fmt.Sprintf(`user_id="%s"`, userID)
 	items, err := s.pb.listAll(ctx, "notifications", filter, "-created")
 	if err != nil {
 		return nil, err
@@ -805,7 +805,7 @@ func (s *Store) DeleteNotification(ctx context.Context, id string) error {
 
 // ClearAllNotifications deletes all notifications for a user.
 func (s *Store) ClearAllNotifications(ctx context.Context, userID string) error {
-	filter := fmt.Sprintf("user_id='%s'", userID)
+	filter := fmt.Sprintf(`user_id="%s"`, userID)
 	items, err := s.pb.listAll(ctx, "notifications", filter, "")
 	if err != nil {
 		return fmt.Errorf("ClearAllNotifications list: %w", err)
@@ -823,7 +823,7 @@ func (s *Store) ClearAllNotifications(ctx context.Context, userID string) error
 
 // MarkAllNotificationsRead marks all notifications for a user as read.
 func (s *Store) MarkAllNotificationsRead(ctx context.Context, userID string) error {
-	filter := fmt.Sprintf("user_id='%s'&&read=false", userID)
+	filter := fmt.Sprintf(`user_id="%s"&&read=false`, userID)
 	items, err := s.pb.listAll(ctx, "notifications", filter, "")
 	if err != nil {
 		return fmt.Errorf("MarkAllNotificationsRead list: %w", err)