新功能(#65): 添加用户订阅限速数据层字段

Co-authored-by: multica-agent <github@multica.ai>

initialize/migrate/database/02153_user_subscribe_speed_limit.down.sql

@@ -0,0 +1,5 @@
+-- Purpose: Rollback user-level speed limit overrides from user_subscribe
+
+ALTER TABLE `user_subscribe`
+  DROP COLUMN IF EXISTS `traffic_limit`,
+  DROP COLUMN IF EXISTS `speed_limit`;