fix: room_store __db name mangling + add singleton

danding_bot/plugins/group_horse_racing/room_store.py

@@ -102,7 +102,7 @@ class RoomStore:
     async def load_rooms(self):
         """Restore active rooms from DB snapshots on startup."""
         await self.ensure_initialized()
-        db = await self.__db if self._db else await self._get_db()
+        db = await self._get_db()
         cursor = await db.execute(
             "SELECT scope, state, created_at, horses, bets, champion_name, tick_count FROM room_snapshots"
         )