feat(models): 组织、用户、角色、客户、会话与消息模型

按组织边界定义数据关系，补齐仓储层与通用工具，迁移脚本可重复执行。

backend/app/models/schemas/user_group.py

@@ -0,0 +1,40 @@
+"""
+Copyright 2024-2026 ChatterMate
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+"""
+
+from pydantic import BaseModel
+from typing import Optional, List
+from uuid import UUID
+from app.models.schemas.user import UserResponse
+
+class UserGroupBase(BaseModel):
+    name: str
+    description: Optional[str] = None
+
+class UserGroupCreate(UserGroupBase):
+    pass
+
+class UserGroupUpdate(UserGroupBase):
+    name: Optional[str] = None
+
+class UserGroupResponse(UserGroupBase):
+    id: UUID
+    organization_id: UUID
+
+    class Config:
+        from_attributes = True
+
+class UserGroupWithUsers(UserGroupResponse):
+    users: List[UserResponse]