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/purchaseLogic.go

@@ -3,7 +3,6 @@ package order
 import (
 	"context"
 	"encoding/json"
-	"math"
 	"strings"
 	"time"
 
@@ -204,14 +203,28 @@ func (l *PurchaseLogic) Purchase(req *types.PurchaseOrderRequest) (resp *types.P
 		return nil, err
 	}
 
-	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,
+		orderType == 1,
+	)
+	if err != nil {
+		l.Errorw("[Purchase] 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
-	// discount amount
-	amount := int64(math.Round(float64(price) * discount))
-	discountAmount := price - amount
+	price := priceResult.OriginalPrice
+	amount := priceResult.PayableBase
+	discountAmount := priceResult.DiscountAmount
 
 	// Validate amount to prevent overflow
 	if amount > MaxOrderAmount {
@@ -306,6 +319,8 @@ func (l *PurchaseLogic) Purchase(req *types.PurchaseOrderRequest) (resp *types.P
 		Price:              price,
 		Amount:             amount,
 		Discount:           discountAmount,
+		PromoRuleId:        priceResult.PromoRuleId,
+		PromoDiscount:      priceResult.PromoDiscount,
 		GiftAmount:         deductionAmount,
 		Coupon:             req.Coupon,
 		CouponDiscount:     coupon,