fix: getDiscount 新人价取值错误 — 始终优先取 new_user_only=false 的实际折扣档

同一 quantity 有两条记录时：
- new_user_only=true (discount=99.64) 是新人限制标记，非实际折扣
- new_user_only=false (discount=35.36) 是实际折扣价格

原逻辑遍历到第一条即返回，导致新人拿到 99.64 而非 35.36。
修复后优先取 new_user_only=false 的条目，仅在无 false 条目时才降级到 new_user_only=true。

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

internal/logic/public/order/getDiscount.go

@@ -2,17 +2,34 @@ package order
 
 import "github.com/perfect-panel/server/internal/types"
 
+// getDiscount returns the discount factor for the given quantity.
+//
+// Design: when a quantity has both a new_user_only=true entry and a new_user_only=false entry,
+// the new_user_only=true entry is a "gating marker" (its discount value is irrelevant).
+// The actual discounted price is always carried by the new_user_only=false entry.
+// Whether the buyer is eligible to use this quantity is enforced by isNewUserOnlyForQuantity
+// before calling this function. Here we simply return the best applicable discount.
+//
+// Priority: new_user_only=false entry beats new_user_only=true entry for the same quantity.
 func getDiscount(discounts []types.SubscribeDiscount, inputMonths int64, isNewUser bool) float64 {
-	for _, discount := range discounts {
-		if discount.NewUserOnly && !isNewUser {
+	var best float64 = 1
+	foundFallback := false
+	for _, d := range discounts {
+		if d.Quantity != inputMonths || d.Discount <= 0 || d.Discount >= 100 {
 			continue
 		}
-		if inputMonths == discount.Quantity && discount.Discount > 0 && discount.Discount < 100 {
-			return discount.Discount / float64(100)
+		factor := d.Discount / float64(100)
+		if !d.NewUserOnly {
+			// Prefer the non-new-user-only tier as the actual price tier.
+			best = factor
+			foundFallback = true
+		} else if !foundFallback && isNewUser {
+			// Fall back to new-user-only tier only when no general tier exists
+			// and the buyer qualifies.
+			best = factor
 		}
 	}
-
-	return 1
+	return best
 }
 
 // isNewUserOnlyForQuantity checks whether the matched discount tier has new_user_only enabled.