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/widget.py

@@ -0,0 +1,37 @@
+"""
+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 import Column, String, ForeignKey, Integer
+from sqlalchemy.orm import relationship
+from uuid import  uuid4
+from sqlalchemy.dialects.postgresql import UUID
+from app.database import Base
+
+
+class Widget(Base):
+    __tablename__ = "widgets"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid4()))
+    name = Column(String, nullable=False)
+    organization_id = Column(UUID(as_uuid=True), ForeignKey(
+        "organizations.id"), nullable=False)
+
+    # Agent relationship
+    agent_id = Column(UUID(as_uuid=True), ForeignKey("agents.id"))
+    agent = relationship("Agent", back_populates="widgets")
+
+    # Relationships
+    organization = relationship("Organization", back_populates="widgets")