新功能(#76): 促销系统下单流程集成

- 新增促销规则/规格促销价/使用记录模型与幂等迁移
- EvaluatePromo 支持 new_user/inactive_user/campaign 规则判定
- purchase/preCreate 接入促销判定：命中促销时跳过百分比折扣
- activate 激活后按 order_no 幂等写入 promo_usage
- 订单模型和响应结构新增 promo_rule_id、promo_discount 字段

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

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

@@ -0,0 +1,39 @@
+SET @column_exists = (
+  SELECT COUNT(*)
+  FROM INFORMATION_SCHEMA.COLUMNS
+  WHERE TABLE_SCHEMA = DATABASE()
+    AND TABLE_NAME = 'order'
+    AND COLUMN_NAME = 'promo_discount'
+);
+
+SET @sql = IF(
+  @column_exists = 1,
+  'ALTER TABLE `order` DROP COLUMN `promo_discount`',
+  'SELECT ''Column promo_discount does not exist in order 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 = 'order'
+    AND COLUMN_NAME = 'promo_rule_id'
+);
+
+SET @sql = IF(
+  @column_exists = 1,
+  'ALTER TABLE `order` DROP COLUMN `promo_rule_id`',
+  'SELECT ''Column promo_rule_id does not exist in order table'''
+);
+
+PREPARE stmt FROM @sql;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+DROP TABLE IF EXISTS `promo_usage`;
+DROP TABLE IF EXISTS `subscribe_promo`;
+DROP TABLE IF EXISTS `promo_rule`;