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/notification-inbox-protocol.md

@@ -27,6 +27,7 @@ List notifications for the current user.
 
 - `limit` (optional, integer, default 20, max 50): number of items per page
 - `cursor` (optional, string): pagination cursor (ISO 8601 timestamp of last item's `created_at`)
+- `locale` (optional, string): requested locale for title/body resolution. Supported values: `zh` (default), `zh_Hant`, `en`. If the requested locale is not available in the notification's i18n dict, falls back to `zh`.
 
 **Response (200)**:
 
@@ -61,6 +62,7 @@ Field rules:
 - `payload`: discriminated union (see Payload section below)
 - `isRead`: boolean
 - `readAt`: ISO 8601 timestamp or `null`
+- `title` and `body`: resolved plain strings based on the `locale` parameter. The database stores these as i18n JSONB objects (`{"zh": "...", "zh_Hant": "...", "en": "..."}`); the API resolves the best match before returning.
 - Results are filtered: `notifications.status = 'published'` and `notifications.deleted_at IS NULL`
 
 ### GET /api/v1/notifications/unread-count
@@ -92,6 +94,10 @@ Mark a single notification as read. Idempotent.
 
 - `notification_id`: UUID of the `user_notifications` record
 
+**Query parameters**:
+
+- `locale` (optional, string): requested locale for title/body resolution (same rules as list endpoint)
+
 **Response (200)**:
 
 ```json