feat: 实现站内通知系统

- 后端: 新增 notifications/user_notifications 表迁移及 ORM 模型
- 后端: 实现 schema/repository/service/router 全套通知 API
  - GET /api/v1/notifications (列表+游标分页)
  - GET /api/v1/notifications/unread-count
  - PATCH /api/v1/notifications/{id}/read (幂等)
  - PATCH /api/v1/notifications/mark-all-read (幂等)
- 后端: payload 使用 Pydantic discriminated union (none/open_route/open_url)
- 后端: 19 个单元测试全部通过
- Flutter: 通知 feature 完整实现 (models/apis/repositories/bloc/UI)
- Flutter: Home 页通知按钮接入真实页面，显示未读 badge
- Flutter: 14 个测试全部通过
- 协议文档: notification-inbox-protocol.md 及错误码注册

backend/src/v1/router.py

@@ -4,6 +4,7 @@ from fastapi import APIRouter
 
 from v1.agent.router import router as agent_router
 from v1.auth.router import router as auth_router
+from v1.notifications.router import router as notifications_router
 from v1.points.router import router as points_router
 from v1.users.router import router as users_router
 
@@ -11,5 +12,6 @@ from v1.users.router import router as users_router
 router = APIRouter(prefix="/api/v1")
 router.include_router(auth_router)
 router.include_router(agent_router)
+router.include_router(notifications_router)
 router.include_router(points_router)
 router.include_router(users_router)