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_item.dart

@@ -0,0 +1,39 @@
+import 'notification_payload.dart';
+
+class NotificationItem {
+  const NotificationItem({
+    required this.id,
+    required this.notificationId,
+    required this.type,
+    required this.title,
+    required this.body,
+    required this.payload,
+    required this.isRead,
+    required this.createdAt,
+    this.readAt,
+  });
+
+  final String id;
+  final String notificationId;
+  final String type;
+  final String title;
+  final String body;
+  final NotificationPayload payload;
+  final bool isRead;
+  final DateTime createdAt;
+  final DateTime? readAt;
+
+  NotificationItem copyWith({bool? isRead, DateTime? readAt}) {
+    return NotificationItem(
+      id: id,
+      notificationId: notificationId,
+      type: type,
+      title: title,
+      body: body,
+      payload: payload,
+      isRead: isRead ?? this.isRead,
+      createdAt: createdAt,
+      readAt: readAt ?? this.readAt,
+    );
+  }
+}