fix: 折扣匹配改为精确匹配 quantity，不再取全局最小值

将 getDiscount 和 isNewUserOnlyForQuantity 的匹配逻辑从
inputMonths >= discount.Quantity 改为 inputMonths == discount.Quantity，
买多少天就用多少天对应的折扣，避免错误配置导致折扣泄漏。
同时增加 discount 值合法性校验（>0 且 <100）。

Co-Authored-By: claude-flow <ruv@ruv.net>

queue/logic/order/activateOrderLogic.go

@@ -1479,15 +1479,11 @@ func (l *ActivateOrderLogic) RedemptionActivate(ctx context.Context, orderInfo *
 
 // isNewUserOnlyForQuantity checks whether the matched discount tier has new_user_only enabled.
 func isNewUserOnlyForQuantity(discounts []internaltypes.SubscribeDiscount, inputQuantity int64) bool {
-	var finalDiscount float64 = 100
-	var newUserOnly bool
-
 	for _, d := range discounts {
-		if inputQuantity >= d.Quantity && d.Discount < finalDiscount {
-			finalDiscount = d.Discount
-			newUserOnly = d.NewUserOnly
+		if inputQuantity == d.Quantity {
+			return d.NewUserOnly
 		}
 	}
 
-	return newUserOnly
+	return false
 }