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>

internal/logic/public/order/preCreateOrderLogic.go

@@ -2,7 +2,6 @@ package order
 
 import (
 	"context"
-	"math"
 
 	commonLogic "github.com/perfect-panel/server/internal/logic/common"
 	"github.com/perfect-panel/server/internal/model/order"
@@ -115,15 +114,28 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 		return nil, errors.Wrapf(xerr.NewErrCode(xerr.SubscribeNewUserOnly), "not a new user")
 	}
 
-	var discount float64 = 1
-	if len(newUserDiscount.Discounts) > 0 {
-		discount = getDiscount(newUserDiscount.Discounts, req.Quantity, newUserDiscount.EligibleForDiscount)
+	priceResult, err := calculatePurchasePrice(
+		l.ctx,
+		l.svcCtx,
+		u.Id,
+		targetSubscribeID,
+		sub.UnitPrice,
+		req.Quantity,
+		newUserDiscount.Discounts,
+		newUserDiscount.EligibleForDiscount,
+		!isSingleModeRenewal,
+	)
+	if err != nil {
+		l.Errorw("[PreCreateOrder] Promo price calculation error",
+			logger.Field("error", err.Error()),
+			logger.Field("user_id", u.Id),
+			logger.Field("subscribe_id", targetSubscribeID),
+		)
+		return nil, err
 	}
-	price := sub.UnitPrice * req.Quantity
-
-	amount := int64(math.Round(float64(price) * discount))
-	discountAmount := price - amount
-	var promoDiscount int64
+	price := priceResult.OriginalPrice
+	amount := priceResult.PayableBase
+	discountAmount := priceResult.DiscountAmount
 	var couponAmount int64
 	if req.Coupon != "" {
 		couponInfo, err := l.svcCtx.CouponModel.FindOneByCode(l.ctx, req.Coupon)
@@ -186,7 +198,7 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 		Price:          price,
 		Amount:         amount,
 		Discount:       discountAmount,
-		PromoDiscount:  promoDiscount,
+		PromoDiscount:  priceResult.PromoDiscount,
 		GiftAmount:     deductionAmount,
 		Coupon:         req.Coupon,
 		CouponDiscount: couponAmount,