feat(M01-B): 增加迁移执行器与旧库只读兼容层

backend/src/db/mysql.ts

@@ -23,3 +23,7 @@ export function toPoolOptions(config: AppConfig): PoolOptions {
     dateStrings: false
   };
 }
+
+export async function closeMySqlPool(pool: MySqlPool): Promise<void> {
+  await pool.end();
+}