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/channels/channel_conversation.py

@@ -0,0 +1,92 @@
+"""
+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 uuid
+
+from sqlalchemy import Column, String, DateTime, ForeignKey, JSON, UniqueConstraint
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+from sqlalchemy.dialects.postgresql import UUID
+
+from app.database import Base
+
+
+class ChannelConversation(Base):
+    """Maps an external channel conversation to a ChatterMate session.
+
+    The generic analog of SlackConversation: one row per
+    (connected account, platform conversation, session). A new session for
+    the same platform conversation (e.g. after the previous one closes)
+    gets a new row.
+    """
+    __tablename__ = "channel_conversations"
+
+    __table_args__ = (
+        UniqueConstraint(
+            'channel_account_id', 'external_conversation_id', 'session_id',
+            name='uq_channel_conversation_session'
+        ),
+    )
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    channel_account_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("channel_accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+    channel_type = Column(String, nullable=False)
+    # Platform conversation id: Telegram chat_id, WhatsApp wa_id,
+    # Messenger PSID, Instagram IGSID, Slack thread key, email thread id, ...
+    external_conversation_id = Column(String, nullable=False, index=True)
+    # Platform user id of the customer (often equals the conversation id)
+    external_user_id = Column(String, nullable=False)
+
+    session_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("session_to_agents.session_id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+
+    # Denormalized for faster webhook-path lookups
+    organization_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("organizations.id", ondelete="CASCADE"),
+        nullable=False
+    )
+    agent_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("agents.id", ondelete="SET NULL"),
+        nullable=True
+    )
+    customer_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("customers.id", ondelete="SET NULL"),
+        nullable=True
+    )
+
+    # Timestamp of the customer's latest inbound message; drives the
+    # 24h (Meta) delivery-window checks.
+    last_inbound_at = Column(DateTime(timezone=True), nullable=True)
+    # Channel-specific extras (message thread ids, profile snapshots, ...)
+    extra = Column(JSON, default=dict)
+
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+    account = relationship("ChannelAccount")
+    session = relationship("SessionToAgent")