修复(#88): 兼容用户级限速迁移

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

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

@@ -1,5 +1,37 @@
 -- 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`;
+SET @column_exists = (
+  SELECT COUNT(*)
+  FROM INFORMATION_SCHEMA.COLUMNS
+  WHERE TABLE_SCHEMA = DATABASE()
+    AND TABLE_NAME = 'user_subscribe'
+    AND COLUMN_NAME = 'traffic_limit'
+);
+
+SET @sql = IF(
+  @column_exists = 1,
+  'ALTER TABLE `user_subscribe` DROP COLUMN `traffic_limit`',
+  'SELECT ''Column traffic_limit does not exist in user_subscribe table'''
+);
+
+PREPARE stmt FROM @sql;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+SET @column_exists = (
+  SELECT COUNT(*)
+  FROM INFORMATION_SCHEMA.COLUMNS
+  WHERE TABLE_SCHEMA = DATABASE()
+    AND TABLE_NAME = 'user_subscribe'
+    AND COLUMN_NAME = 'speed_limit'
+);
+
+SET @sql = IF(
+  @column_exists = 1,
+  'ALTER TABLE `user_subscribe` DROP COLUMN `speed_limit`',
+  'SELECT ''Column speed_limit does not exist in user_subscribe table'''
+);
+
+PREPARE stmt FROM @sql;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;