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/chat_history.py

@@ -0,0 +1,71 @@
+"""
+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, Integer, String, JSON, ForeignKey, TIMESTAMP, func, UUID, Interval, Float, Index, text
+from sqlalchemy.orm import relationship
+from app.core.encryption import EncryptedText
+from app.database import Base
+
+
+class ChatHistory(Base):
+    __tablename__ = "chat_history"
+    __table_args__ = (
+        # Every read of a conversation filters by session_id and orders by
+        # (created_at, id) — the inbox previews, get_session_history, the AI's
+        # context window. Declared here as well as in the migration so autogenerate
+        # never proposes dropping it and the sqlite test schema matches Postgres.
+        Index('ix_chat_history_session_created', 'session_id',
+              text('created_at DESC'), text('id DESC')),
+        # Delivery receipts look their message up by platform id
+        Index('ix_chat_history_org_external_message', 'organization_id', 'external_message_id',
+              postgresql_where=text('external_message_id IS NOT NULL'),
+              sqlite_where=text('external_message_id IS NOT NULL')),
+    )
+
+    id = Column(Integer, primary_key=True, index=True)
+    organization_id = Column(UUID(as_uuid=True), ForeignKey(
+        "organizations.id", ondelete="SET NULL"), nullable=True)
+    user_id = Column(UUID(as_uuid=True), ForeignKey(
+        "users.id", ondelete="SET NULL"), nullable=True)
+    customer_id = Column(UUID(as_uuid=True), ForeignKey(
+        "customers.id", ondelete="SET NULL"), nullable=True)
+    agent_id = Column(UUID(as_uuid=True), ForeignKey(
+        "agents.id", ondelete="SET NULL"), nullable=True)
+    session_id = Column(UUID(as_uuid=True), ForeignKey("session_to_agents.session_id"), nullable=True)
+    # Encrypted at rest — see app.core.encryption. Storage stays text, so rows
+    # written before the change read back as plaintext until the backfill lands.
+    message = Column(EncryptedText, nullable=False)
+    # 'user', 'bot', or 'agent'
+    message_type = Column(String, nullable=False)
+    attributes = Column(JSON, default={})
+    sentiment_label = Column(String, nullable=True)  # 'positive', 'neutral', 'negative'
+    sentiment_score = Column(Float, nullable=True)  # -1.0 to 1.0
+    # Channel messages only: the platform's id for this message (WhatsApp wamid)
+    # and its latest delivery receipt — app.channels.base.DeliveryState values.
+    external_message_id = Column(String, nullable=True)
+    channel_status = Column(String(16), nullable=True)
+    channel_status_at = Column(TIMESTAMP(timezone=True), 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
+    user = relationship("User", back_populates="chat_histories")
+    customer = relationship("Customer", back_populates="chat_histories")
+    agent = relationship("Agent", back_populates="chat_histories")
+    organization = relationship("Organization", back_populates="chat_histories")
+    session_assignment = relationship("SessionToAgent")
+