fix: getDiscount 新人取折扣最大档，非新人取最贵档

- 新人：遍历所有匹配 quantity 的条目，取 discount 值最小的（折扣力度最大）
- 非新人：仅看 new_user_only=false 的条目，取 discount 值最大的（最接近原价）

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

internal/logic/public/order/getDiscount.go

@@ -4,32 +4,34 @@ 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.
+//   - 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.
 func getDiscount(discounts []types.SubscribeDiscount, inputMonths int64, isNewUser bool) float64 {
-	var best float64 = 1
-	foundFallback := false
+	best := float64(-1)
 	for _, d := range discounts {
 		if d.Quantity != inputMonths || d.Discount <= 0 || d.Discount >= 100 {
 			continue
 		}
-		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
+		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)
+			if best < 0 || d.Discount > best {
+				best = d.Discount
 			}
 		}
-	return best
+	}
+	if best < 0 {
+		return 1
+	}
+	return best / float64(100)
 }
 
 // isNewUserOnlyForQuantity checks whether the matched discount tier has new_user_only enabled.