refactor: 修复database单例多次初始化的问题，改变instance默认返回实例的类型，缩短db相关函数调用时的object名

src/plugins/chat/chat_stream.py

@@ -111,11 +111,11 @@ class ChatManager:
 
     def _ensure_collection(self):
         """确保数据库集合存在并创建索引"""
-        if "chat_streams" not in self.db.db.list_collection_names():
-            self.db.db.create_collection("chat_streams")
+        if "chat_streams" not in self.db.list_collection_names():
+            self.db.create_collection("chat_streams")
             # 创建索引
-            self.db.db.chat_streams.create_index([("stream_id", 1)], unique=True)
-            self.db.db.chat_streams.create_index(
+            self.db.chat_streams.create_index([("stream_id", 1)], unique=True)
+            self.db.chat_streams.create_index(
                 [("platform", 1), ("user_info.user_id", 1), ("group_info.group_id", 1)]
             )
 
@@ -168,7 +168,7 @@ class ChatManager:
             return stream
 
         # 检查数据库中是否存在
-        data = self.db.db.chat_streams.find_one({"stream_id": stream_id})
+        data = self.db.chat_streams.find_one({"stream_id": stream_id})
         if data:
             stream = ChatStream.from_dict(data)
             # 更新用户信息和群组信息
@@ -204,7 +204,7 @@ class ChatManager:
     async def _save_stream(self, stream: ChatStream):
         """保存聊天流到数据库"""
         if not stream.saved:
-            self.db.db.chat_streams.update_one(
+            self.db.chat_streams.update_one(
                 {"stream_id": stream.stream_id}, {"$set": stream.to_dict()}, upsert=True
             )
             stream.saved = True
@@ -216,7 +216,7 @@ class ChatManager:
 
     async def load_all_streams(self):
         """从数据库加载所有聊天流"""
-        all_streams = self.db.db.chat_streams.find({})
+        all_streams = self.db.chat_streams.find({})
         for data in all_streams:
             stream = ChatStream.from_dict(data)
             self.streams[stream.stream_id] = stream