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

docs/protocols/common/http-error-codes.md

@@ -77,6 +77,12 @@ This document is the source of truth for backend RFC7807 `code` values consumed
 | `AVATAR_SIGNED_URL_FAILED` | 502 | Backend failed to generate avatar signed upload URL | Show retry toast and keep previous avatar |
 | `AVATAR_UPLOAD_FAILED` | 502 | Backend failed to upload avatar bytes to storage | Show retry toast and keep previous avatar |
 
+## Notification
+
+| code | status | meaning | frontend handling |
+|---|---:|---|---|
+| `NOTIFICATION_NOT_FOUND` | 404 | Notification not found or not owned by current user | Show not-found message and refresh list |
+
 ## Global
 
 | code | status | meaning | frontend handling |