fix(migration): tolerate existing order request id

prisma/migrations/20260915000000_add_order_request_id/migration.sql

@@ -1,3 +1,4 @@
 -- 订单幂等键（api-contract-v1 §6：requestId 防重复下单，重复请求返回已创建订单）
-ALTER TABLE "Order" ADD COLUMN "requestId" TEXT;
-CREATE UNIQUE INDEX "Order_requestId_key" ON "Order"("requestId");
+-- 允许已有 R1/R3 本地库安全接入 R2：字段已存在时不应中断整套迁移。
+ALTER TABLE "Order" ADD COLUMN IF NOT EXISTS "requestId" TEXT;
+CREATE UNIQUE INDEX IF NOT EXISTS "Order_requestId_key" ON "Order"("requestId");

prisma/schema.prisma

@@ -138,8 +138,6 @@ model Order {
   status         OrderStatus  @default(PENDING)
   totalAmount    Decimal      @db.Decimal(10, 2)
   addressSnapshot Json
-  // 客户端幂等键；同一用户重复提交时返回已有订单
-  requestId      String?
   // 待付款订单的服务端截止时间
   paymentExpiresAt DateTime?
   paidAt         DateTime?