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/file_attachment.py

@@ -0,0 +1,46 @@
+"""
+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 sqlalchemy import Column, Integer, String, BigInteger, ForeignKey, TIMESTAMP, func, UUID
+from sqlalchemy.orm import relationship
+from app.database import Base
+
+
+class FileAttachment(Base):
+    __tablename__ = "file_attachments"
+
+    id = Column(Integer, primary_key=True, index=True)
+    file_url = Column(String, nullable=False)  # AWS S3 URL or local path
+    filename = Column(String, nullable=False)  # Original filename
+    content_type = Column(String, nullable=False)  # MIME type
+    file_size = Column(BigInteger, nullable=False)  # File size in bytes
+    
+    # Links to chat history message
+    chat_history_id = Column(Integer, ForeignKey("chat_history.id", ondelete="CASCADE"), nullable=True)
+    
+    # Additional metadata
+    organization_id = Column(UUID(as_uuid=True), ForeignKey("organizations.id", ondelete="CASCADE"), nullable=False)
+    uploaded_by_user_id = Column(UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True)
+    uploaded_by_customer_id = Column(UUID(as_uuid=True), ForeignKey("customers.id", ondelete="SET NULL"), nullable=True)
+    
+    created_at = Column(TIMESTAMP(timezone=True), server_default=func.now())
+    updated_at = Column(TIMESTAMP(timezone=True), server_default=func.now(), onupdate=func.now())
+
+    # Relationships
+    chat_history = relationship("ChatHistory", backref="attachments")
+    organization = relationship("Organization")
+    uploaded_by_user = relationship("User", foreign_keys=[uploaded_by_user_id])
+    uploaded_by_customer = relationship("Customer", foreign_keys=[uploaded_by_customer_id])