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

apps/lib/features/notifications/data/models/notification_payload.dart

@@ -0,0 +1,43 @@
+sealed class NotificationPayload {
+  const NotificationPayload();
+}
+
+final class NotificationPayloadNone extends NotificationPayload {
+  const NotificationPayloadNone();
+}
+
+final class NotificationPayloadRoute extends NotificationPayload {
+  const NotificationPayloadRoute({
+    required this.route,
+    this.entityId,
+    this.tab,
+  });
+
+  final String route;
+  final String? entityId;
+  final String? tab;
+}
+
+final class NotificationPayloadUrl extends NotificationPayload {
+  const NotificationPayloadUrl({required this.url});
+
+  final String url;
+}
+
+NotificationPayload parseNotificationPayload(Map<String, dynamic> json) {
+  final action = json['action'];
+  switch (action) {
+    case 'open_route':
+      return NotificationPayloadRoute(
+        route: json['route'] as String? ?? '',
+        entityId: json['entityId'] as String?,
+        tab: json['tab'] as String?,
+      );
+    case 'open_url':
+      return NotificationPayloadUrl(url: json['url'] as String? ?? '');
+    case 'none':
+      return const NotificationPayloadNone();
+    default:
+      return const NotificationPayloadNone();
+  }
+}