fix: getDiscount 新人取折扣最大档，非新人取折扣最小档

- 新人：所有匹配 quantity 的条目中取 discount 值最小的（折扣力度最大）
- 非新人：所有匹配 quantity 的条目中取 discount 值最大的（折扣力度最小，接近原价）
- 不再按 new_user_only 过滤，由 isNewUserOnlyForQuantity 在上层控制准入

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

internal/logic/public/order/getDiscount.go

@@ -4,25 +4,21 @@ import "github.com/perfect-panel/server/internal/types"
 
 // getDiscount returns the discount factor for the given quantity.
 //
-//   - New user: pick the tier with the lowest discount value (best deal) among all matching tiers.
-//   - Non-new user: pick the tier with the highest discount value (most expensive / least discount)
-//     among tiers where new_user_only=false only.
+//   - New user: pick the tier with the lowest discount value (biggest saving).
+//   - Non-new user: pick the tier with the highest discount value (least saving / closest to full price).
 func getDiscount(discounts []types.SubscribeDiscount, inputMonths int64, isNewUser bool) float64 {
 	best := float64(-1)
 	for _, d := range discounts {
 		if d.Quantity != inputMonths || d.Discount <= 0 || d.Discount >= 100 {
 			continue
 		}
-		if !isNewUser && d.NewUserOnly {
-			continue
-		}
 		if isNewUser {
 			// lowest discount value = biggest saving
 			if best < 0 || d.Discount < best {
 				best = d.Discount
 			}
 		} else {
-			// highest discount value = least discount (closest to original price)
+			// highest discount value = least saving (closest to original price)
 			if best < 0 || d.Discount > best {
 				best = d.Discount
 			}