新功能(#69): 用户级限速覆盖 — 数据层 + 核心逻辑 + 管理接口

- 新增 migration 02153: user_subscribe 表添加 speed_limit、traffic_limit 列（幂等）
- model 层 Subscribe/SubscribeDetails 新增用户级限速字段（*string 类型支持 nil 回退）
- server 用户列表和管理员详情接口支持用户级限速覆盖优先级计算
- 更新管理员订阅更新接口支持写入/清除用户级限速

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

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

@@ -1,5 +1,35 @@
--- Purpose: Rollback user-level speed limit overrides from user_subscribe
+SET @traffic_limit_exists = (
+  SELECT COUNT(*)
+  FROM INFORMATION_SCHEMA.COLUMNS
+  WHERE TABLE_SCHEMA = DATABASE()
+    AND TABLE_NAME = 'user_subscribe'
+    AND COLUMN_NAME = 'traffic_limit'
+);
 
-ALTER TABLE `user_subscribe`
-  DROP COLUMN IF EXISTS `traffic_limit`,
-  DROP COLUMN IF EXISTS `speed_limit`;
+SET @traffic_limit_sql = IF(
+  @traffic_limit_exists = 1,
+  'ALTER TABLE `user_subscribe` DROP COLUMN `traffic_limit`',
+  'SELECT 1'
+);
+
+PREPARE traffic_limit_stmt FROM @traffic_limit_sql;
+EXECUTE traffic_limit_stmt;
+DEALLOCATE PREPARE traffic_limit_stmt;
+
+SET @speed_limit_exists = (
+  SELECT COUNT(*)
+  FROM INFORMATION_SCHEMA.COLUMNS
+  WHERE TABLE_SCHEMA = DATABASE()
+    AND TABLE_NAME = 'user_subscribe'
+    AND COLUMN_NAME = 'speed_limit'
+);
+
+SET @speed_limit_sql = IF(
+  @speed_limit_exists = 1,
+  'ALTER TABLE `user_subscribe` DROP COLUMN `speed_limit`',
+  'SELECT 1'
+);
+
+PREPARE speed_limit_stmt FROM @speed_limit_sql;
+EXECUTE speed_limit_stmt;
+DEALLOCATE PREPARE speed_limit_stmt;