feat(notification): 通知标题和正文支持多语言

- 通知静态配置支持 title/body i18n
- 前端通知列表和详情页展示本地化内容
- 新增数据库迁移脚本
- 更新通知协议文档

docs/protocols/notification/static-notification-sync-protocol.md

@@ -41,8 +41,14 @@ notification:
   type: system
   status: published
   published_at: 2026-04-10T08:00:00Z
-  title: 新用户欢迎通知
-  body: 你已获得注册奖励，可前往积分中心查看。
+  title:
+    zh: 新用户欢迎通知
+    zh_Hant: 新用戶歡迎通知
+    en: Welcome
+  body:
+    zh: 你已获得注册奖励，可前往积分中心查看。
+    zh_Hant: 你已獲得註冊獎勵，可前往積分中心查看。
+    en: You have received a registration reward. Check your points.
   payload:
     action: open_route
     route: /points
@@ -60,8 +66,8 @@ targets:
 - `status`: required, one of `draft`, `published`, `revoked`
 - `deleted`: optional, boolean, default `false`, soft-delete this notification
 - `published_at`: optional ISO 8601 timestamp
-- `title`: required, non-empty string
-- `body`: required, non-empty string
+- `title`: required, non-empty dict mapping locale codes to translated strings. Must include at least `zh`. Supported keys: `zh`, `zh_Hant`, `en`.
+- `body`: required, non-empty dict mapping locale codes to translated strings. Must include at least `zh`. Supported keys: `zh`, `zh_Hant`, `en`.
 - `payload`: required, must follow the notification payload protocol
 
 ### targets