新功能(#74): 新增促销系统模型层

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

initialize/migrate/database/02154_promo_system.down.sql

@@ -0,0 +1,7 @@
+ALTER TABLE `order`
+  DROP COLUMN IF EXISTS `promo_discount`,
+  DROP COLUMN IF EXISTS `promo_rule_id`;
+
+DROP TABLE IF EXISTS `promo_usage`;
+DROP TABLE IF EXISTS `subscribe_promo`;
+DROP TABLE IF EXISTS `promo_rule`;