新功能(#78): 预算订单 API 返回 promo_discount 字段

PreOrderResponse 新增 promo_discount 字段，未接入促销时默认返回 0。
促销命中逻辑待 HIF-76 接入后生效。

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

apis/types.api

@@ -708,6 +708,7 @@ type (
 		Price          int64  `json:"price"`
 		Amount         int64  `json:"amount"`
 		Discount       int64  `json:"discount"`
+		PromoDiscount  int64  `json:"promo_discount"`
 		GiftAmount     int64  `json:"gift_amount"`
 		Coupon         string `json:"coupon"`
 		CouponDiscount int64  `json:"coupon_discount"`

internal/logic/public/order/preCreateOrderLogic.go

@@ -123,6 +123,7 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 
 	amount := int64(math.Round(float64(price) * discount))
 	discountAmount := price - amount
+	var promoDiscount int64
 	var couponAmount int64
 	if req.Coupon != "" {
 		couponInfo, err := l.svcCtx.CouponModel.FindOneByCode(l.ctx, req.Coupon)
@@ -185,6 +186,7 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 		Price:          price,
 		Amount:         amount,
 		Discount:       discountAmount,
+		PromoDiscount:  promoDiscount,
 		GiftAmount:     deductionAmount,
 		Coupon:         req.Coupon,
 		CouponDiscount: couponAmount,

internal/types/types.go

@@ -2012,6 +2012,7 @@ type PreOrderResponse struct {
 	Price          int64  `json:"price"`
 	Amount         int64  `json:"amount"`
 	Discount       int64  `json:"discount"`
+	PromoDiscount  int64  `json:"promo_discount"`
 	GiftAmount     int64  `json:"gift_amount"`
 	Coupon         string `json:"coupon"`
 	CouponDiscount int64  `json:"coupon_discount"`