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

backend/alembic/versions/20260428_0001_notification_title_body_i18n.py

@@ -0,0 +1,51 @@
+"""Convert notification title/body from text to jsonb (i18n dict).
+
+title and body become jsonb objects keyed by locale code:
+  {"zh": "欢迎来到觅爻", "zh_Hant": "...", "en": "..."}
+
+Existing data is wrapped under the "zh" key (simplified Chinese default).
+
+Revision ID: 20260428_0001
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "20260428_0001"
+down_revision = "20260427_0002"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute(
+        """
+        ALTER TABLE notifications
+            ALTER COLUMN title TYPE jsonb USING jsonb_build_object('zh', title),
+            ALTER COLUMN body  TYPE jsonb USING jsonb_build_object('zh', body);
+        """
+    )
+
+    op.execute(
+        """
+        ALTER TABLE notifications DROP CONSTRAINT IF EXISTS ck_notifications_payload_object;
+        """
+    )
+
+    op.execute(
+        """
+        ALTER TABLE notifications
+            ADD CONSTRAINT ck_notifications_payload_object
+            CHECK (jsonb_typeof(payload) = 'object');
+        """
+    )
+
+
+def downgrade() -> None:
+    op.execute(
+        """
+        ALTER TABLE notifications
+            ALTER COLUMN title TYPE text USING COALESCE(title ->> 'zh', ''),
+            ALTER COLUMN body  TYPE text USING COALESCE(body  ->> 'zh', '');
+        """
+    )