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/bookstore/bookstore.go

@@ -247,3 +247,14 @@ type ImportFileStore interface {
 	// GetImportChapters retrieves the pre-parsed chapters JSON.
 	GetImportChapters(ctx context.Context, key string) ([]byte, error)
 }
+
+// NotificationStore manages per-user in-app notifications.
+// Always wired directly to the concrete *storage.Store so it works
+// regardless of whether the Asynq task-queue wrapper is in use.
+type NotificationStore interface {
+	ListNotifications(ctx context.Context, userID string, limit int) ([]map[string]any, error)
+	MarkNotificationRead(ctx context.Context, id string) error
+	MarkAllNotificationsRead(ctx context.Context, userID string) error
+	DeleteNotification(ctx context.Context, id string) error
+	ClearAllNotifications(ctx context.Context, userID string) error
+}