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/pagination.py

@@ -0,0 +1,37 @@
+"""
+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.
+"""
+
+import math
+
+from pydantic import BaseModel
+
+
+class Pagination(BaseModel):
+    """Standard list-endpoint pagination block (shape matches the knowledge
+    API's inline dicts: total / page / page_size / total_pages)."""
+    total: int
+    page: int
+    page_size: int
+    total_pages: int
+
+    @classmethod
+    def build(cls, total: int, page: int, page_size: int) -> "Pagination":
+        return cls(
+            total=total,
+            page=page,
+            page_size=page_size,
+            total_pages=max(1, math.ceil(total / page_size)) if total else 0,
+        )