refactor(room_store): 将上次马名存储从内存迁移至数据库

- 移除内存字典 `_last_horse_names`，改为使用 SQLite 表 `user_horse_names`
- 修改 `get_last_horse_name` 和 `set_last_horse_name` 方法以操作数据库
- 提升数据持久化能力，防止重启后数据丢失

danding_bot/plugins/group_horse_racing/room_store.py

@@ -13,7 +13,6 @@ class RoomStore:
         self.config = config
         self.rooms: dict[str, Room] = {}
         self._locks: dict[str, asyncio.Lock] = {}
-        self._last_horse_names: dict[str, str] = {}
         self.db_path = Path(config.RACE_DB_FILE)
         self.db_path.parent.mkdir(parents=True, exist_ok=True)
         self._init_db()
@@ -48,6 +47,13 @@ class RoomStore:
             )
         """)
 
+        cursor.execute("""
+            CREATE TABLE IF NOT EXISTS user_horse_names (
+                user_id TEXT PRIMARY KEY,
+                horse_name TEXT NOT NULL
+            )
+        """)
+
         conn.commit()
         conn.close()
 
@@ -74,10 +80,22 @@ class RoomStore:
             del self.rooms[scope]
 
     def get_last_horse_name(self, user_id: str) -> Optional[str]:
-        return self._last_horse_names.get(user_id)
+        conn = sqlite3.connect(self.db_path)
+        cursor = conn.cursor()
+        cursor.execute("SELECT horse_name FROM user_horse_names WHERE user_id = ?", (user_id,))
+        row = cursor.fetchone()
+        conn.close()
+        return row[0] if row else None
 
     def set_last_horse_name(self, user_id: str, horse_name: str):
-        self._last_horse_names[user_id] = horse_name
+        conn = sqlite3.connect(self.db_path)
+        cursor = conn.cursor()
+        cursor.execute(
+            "INSERT OR REPLACE INTO user_horse_names (user_id, horse_name) VALUES (?, ?)",
+            (user_id, horse_name),
+        )
+        conn.commit()
+        conn.close()
 
     def _save_snapshot(self, room: Room):
         """Save room snapshot to database."""