修复(#73): 修正促销迁移表结构 — subscribe_promo 加 quantity + 索引优化

- subscribe_promo 新增 quantity 列，唯一键改为 (subscribe_id, quantity, promo_rule_id)
- promo_rule 索引合并为 idx_enabled_priority_deleted (enabled, deleted_at, priority DESC)

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

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

@@ -11,19 +11,19 @@ CREATE TABLE IF NOT EXISTS `promo_rule` (
   `updated_at` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
   `deleted_at` DATETIME DEFAULT NULL COMMENT '软删除时间',
   PRIMARY KEY (`id`),
-  KEY `idx_enabled_priority` (`enabled`, `priority` DESC),
-  KEY `idx_deleted_at` (`deleted_at`)
+  KEY `idx_enabled_priority_deleted` (`enabled`, `deleted_at`, `priority` DESC)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='促销规则表';
 
 CREATE TABLE IF NOT EXISTS `subscribe_promo` (
   `id` BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
   `subscribe_id` BIGINT UNSIGNED NOT NULL COMMENT '套餐规格 ID',
+  `quantity` INT NOT NULL DEFAULT 0 COMMENT '购买数量',
   `promo_rule_id` BIGINT UNSIGNED NOT NULL COMMENT '促销规则 ID',
   `promo_price` BIGINT NOT NULL DEFAULT 0 COMMENT '该规格在此规则下的优惠价（分）',
   `created_at` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
   `updated_at` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
   PRIMARY KEY (`id`),
-  UNIQUE KEY `uk_subscribe_rule` (`subscribe_id`, `promo_rule_id`),
+  UNIQUE KEY `uk_subscribe_qty_rule` (`subscribe_id`, `quantity`, `promo_rule_id`),
   KEY `idx_promo_rule_id` (`promo_rule_id`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='规格促销价表';