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/permission.py

@@ -0,0 +1,86 @@
+"""
+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, Table, ForeignKey
+from app.database import Base
+
+# Junction table for role-permission many-to-many relationship
+role_permissions = Table(
+    'role_permissions',
+    Base.metadata,
+    Column('role_id', Integer, ForeignKey('roles.id')),
+    Column('permission_id', Integer, ForeignKey('permissions.id'))
+)
+
+
+# What a newly seeded "Agent" role can do: work the chats assigned to them, pick
+# up whatever the AI is still handling, and look up who they are talking to.
+#
+# Named here rather than spelled out in each signup handler because there are
+# two of them — community (api/organizations.py) and hosted (enterprise
+# signup) — and they drifted. Hosted seeded only the first two for a year, so
+# its agents could not see the unclaimed queue or open People at all.
+DEFAULT_AGENT_ROLE_PERMISSIONS = (
+    "view_assigned_chats",
+    "manage_assigned_chats",
+    "view_unassigned_chats",
+    "view_people",
+)
+
+
+class Permission(Base):
+    __tablename__ = "permissions"
+
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String, unique=True, nullable=False)
+    description = Column(String)
+
+    # Define default permissions
+    @classmethod
+    def default_permissions(cls):
+        return [
+            ("view_all", "Can view all resources"),
+            ("manage_users", "Can manage users"),
+            ("manage_roles", "Can manage roles"),
+            ("manage_agents", "Can manage chat agents"),
+            ("view_agents", "Can view chat agents"),
+            ("view_analytics", "Can view analytics"),
+            ("view_assigned_chats", "Can view assigned chats only"),
+            ("manage_assigned_chats", "Can manage assigned chats"),
+            # The unclaimed queue: AI-handled sessions nobody has taken yet.
+            # Separate from view_assigned_chats so widening an agent's inbox to
+            # the queue never silently exposes another agent's conversations.
+            ("view_unassigned_chats", "Can view unassigned AI chats"),
+            ("manage_knowledge", "Can manage knowledge base"),
+            ("view_knowledge", "Can view knowledge base"),
+            ("manage_ai_config", "Can manage AI configuration"),
+            ("view_ai_config", "Can view AI configuration"),
+            ("view_all_chats", "Can view all chat history"),
+            ("manage_all_chats", "Can manage all chat sessions"),
+            # The people directory, decoupled from chat visibility: an agent can
+            # look up who they are talking to without being able to read every
+            # other agent's conversations.
+            ("view_people", "Can view the people directory"),
+            ("manage_organization", "Can manage organization settings"),
+            ("view_organization", "Can view organization details"),
+            ("manage_subscription", "Can manage subscription plans and billing"),
+            ("view_subscription", "Can view subscription details"),
+            ("view_tickets", "Can view tickets"),
+            ("manage_tickets", "Can manage tickets"),
+            ("approve_ticket_actions", "Can approve AI-proposed ticket resolutions"),
+            ("manage_ticket_connectors", "Can manage ticket investigation connectors"),
+            ("super_admin", "Has all permissions")
+        ]