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>

internal/logic/public/portal/tool.go

@@ -7,17 +7,15 @@ import (
 )
 
 func getDiscount(discounts []types.SubscribeDiscount, inputMonths int64, isNewUser bool) float64 {
-	var finalDiscount float64 = 100
-
 	for _, discount := range discounts {
 		if discount.NewUserOnly && !isNewUser {
 			continue
 		}
-		if inputMonths >= discount.Quantity && discount.Discount < finalDiscount {
-			finalDiscount = discount.Discount
+		if inputMonths == discount.Quantity && discount.Discount > 0 && discount.Discount < 100 {
+			return discount.Discount / float64(100)
 		}
 	}
-	return finalDiscount / float64(100)
+	return 1
 }
 
 func calculateCoupon(amount int64, couponInfo *coupon.Coupon) int64 {