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/relationship_manager.py

@@ -169,7 +169,7 @@ class RelationshipManager:
     async def load_all_relationships(self):
         """加载所有关系对象"""
         db = Database.get_instance()
-        all_relationships = db.db.relationships.find({})
+        all_relationships = db.relationships.find({})
         for data in all_relationships:
             await self.load_relationship(data)
 
@@ -177,7 +177,7 @@ class RelationshipManager:
         """每5分钟自动保存一次关系数据"""
         db = Database.get_instance()
         # 获取所有关系记录
-        all_relationships = db.db.relationships.find({})
+        all_relationships = db.relationships.find({})
         # 依次加载每条记录
         for data in all_relationships:
             await self.load_relationship(data)
@@ -207,7 +207,7 @@ class RelationshipManager:
         saved = relationship.saved
 
         db = Database.get_instance()
-        db.db.relationships.update_one(
+        db.relationships.update_one(
             {'user_id': user_id, 'platform': platform},
             {'$set': {
                 'platform': platform,