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/ticket_activity.py

@@ -0,0 +1,88 @@
+"""
+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 enum
+import uuid
+
+from sqlalchemy import JSON, Boolean, Column, DateTime, ForeignKey, Index, String, Text
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import expression, func
+
+from app.database import Base
+
+
+from app.models.ticket import _ValueStrEnum
+
+
+class TicketActivityType(_ValueStrEnum):
+    COMMENT = "comment"
+    STATUS_CHANGE = "status_change"
+    ASSIGNMENT = "assignment"
+    PRIORITY_CHANGE = "priority_change"
+    AI_TRIAGE = "ai_triage"
+    AI_INVESTIGATION_STARTED = "ai_investigation_started"
+    AI_INVESTIGATION_COMPLETED = "ai_investigation_completed"
+    AI_RESOLUTION_PROPOSED = "ai_resolution_proposed"
+    AI_RESOLUTION_APPROVED = "ai_resolution_approved"
+    AI_RESOLUTION_REJECTED = "ai_resolution_rejected"
+    CUSTOMER_NOTIFIED = "customer_notified"
+    CUSTOMER_REPLIED = "customer_replied"
+    CUSTOMER_LINKED = "customer_linked"
+    CSAT_REQUESTED = "csat_requested"
+    CSAT_RECEIVED = "csat_received"
+    REOPENED = "reopened"
+    JIRA_ESCALATED = "jira_escalated"
+
+
+class TicketActorType(_ValueStrEnum):
+    USER = "user"
+    AI = "ai"
+    CUSTOMER = "customer"
+    SYSTEM = "system"
+
+
+class TicketActivity(Base):
+    """Append-only ticket feed — doubles as comments and the audit trail of
+    every AI/system/human action. Never updated or deleted in place.
+    """
+    __tablename__ = "ticket_activities"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    ticket_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("tickets.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    activity_type = Column(String, nullable=False)
+    actor_type = Column(String, nullable=False, default=TicketActorType.SYSTEM)
+    actor_user_id = Column(UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True)
+    # Comment text or the human-readable line for audit events.
+    body = Column(Text, nullable=True)
+    # Internal note vs customer-visible comment.
+    is_internal = Column(Boolean, nullable=False, default=True, server_default=expression.true())
+    # Structured context: {"from": "open", "to": "investigating"},
+    # {"run_id": ...}, {"channel": "web"}, ...
+    activity_metadata = Column(JSON, nullable=True)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    ticket = relationship("Ticket", back_populates="activities")
+    actor_user = relationship("User")
+
+    __table_args__ = (
+        Index("ix_ticket_activities_ticket_created", "ticket_id", "created_at"),
+    )