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/repositories/group.py

@@ -0,0 +1,28 @@
+"""
+Copyright 2024-2026 ReplyPier
+
+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 sqlalchemy.orm import Session, joinedload
+from app.models.user import UserGroup
+
+class GroupRepository:
+    def __init__(self, db: Session):
+        self.db = db
+
+    def get_group_with_users(self, group_id: str):
+        return self.db.query(UserGroup)\
+            .options(joinedload(UserGroup.users))\
+            .filter(UserGroup.id == group_id)\
+            .first()