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/cmd/backend/main.go

@@ -199,6 +199,7 @@ func run() error {
 			BookWriter:        store,
 			AIJobStore:        store,
 			BookAdminStore:    store,
+			NotificationStore: store,
 			Log:               log,
 		},
 	)