新增店员等级排序功能

- 添加数据库迁移脚本，为 play_clerk_level_info 表新增 order_number 字段
- 更新测试数据种子，设置默认等级的排序号
- 新增店员用户API测试，验证按等级排序号和在线状态的排序逻辑

play-admin/src/main/resources/db/migration/V14__add_clerk_level_order_number.sql

@@ -0,0 +1,20 @@
+SET @column_exists := (
+    SELECT COUNT(*)
+    FROM information_schema.COLUMNS
+    WHERE TABLE_SCHEMA = DATABASE()
+      AND TABLE_NAME = 'play_clerk_level_info'
+      AND COLUMN_NAME = 'order_number'
+);
+
+SET @ddl := IF(
+    @column_exists = 0,
+    'ALTER TABLE `play_clerk_level_info` ADD COLUMN `order_number` bigint NULL COMMENT ''等级排序号''',
+    'SELECT 1'
+);
+
+PREPARE stmt FROM @ddl;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+UPDATE `play_clerk_level_info`
+SET `order_number` = COALESCE(`order_number`, `level`);