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/agent_channel_config.py

@@ -0,0 +1,58 @@
+"""
+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.
+"""
+
+import uuid
+
+from sqlalchemy import Column, Boolean, DateTime, ForeignKey, UniqueConstraint
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+from sqlalchemy.dialects.postgresql import UUID
+
+from app.database import Base
+
+
+class AgentChannelConfig(Base):
+    """Routes a connected channel account to the AI agent that answers it.
+
+    One config per channel account: every inbound conversation on that
+    account is handled by the mapped agent.
+    """
+    __tablename__ = "agent_channel_configs"
+
+    __table_args__ = (
+        UniqueConstraint('channel_account_id', name='uq_agent_channel_config_account'),
+    )
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    agent_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("agents.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+    channel_account_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("channel_accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+    is_active = Column(Boolean, nullable=False, default=True)
+
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+    agent = relationship("Agent")
+    account = relationship("ChannelAccount")