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/utils/client_ip.py

@@ -0,0 +1,28 @@
+"""
+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.
+
+The client's IP for keying (rate limits, telling visitors apart). Our nginx
+APPENDS the real client IP to any X-Forwarded-For the client sent, so only
+the RIGHTMOST entry is trustworthy - the first one can be anything.
+"""
+
+from fastapi import Request
+
+
+def forwarded_client_ip(request: Request) -> str:
+    forwarded = request.headers.get("x-forwarded-for")
+    if forwarded:
+        return forwarded.split(",")[-1].strip()
+    return request.client.host if request.client else "unknown"