feat: 单订阅模式下赠送订阅自动合并，保持token不变

- purchaseLogic: 允许只有赠送订阅(order_id=0)的用户在SingleModel下新购
- activateOrderLogic: NewPurchase激活时检测赠送订阅，在原有订阅上延长到期时间而非创建新记录

internal/logic/public/order/purchaseLogic.go

@@ -66,7 +66,15 @@ func (l *PurchaseLogic) Purchase(req *types.PurchaseOrderRequest) (resp *types.P
 		return nil, errors.Wrapf(xerr.NewErrCode(xerr.DatabaseQueryError), "find user subscription error: %v", err.Error())
 	}
 	if l.svcCtx.Config.Subscribe.SingleModel {
-		if len(userSub) > 0 {
+		// 检查是否有非赠送的活跃订阅（order_id > 0 表示付费订阅）
+		hasPaidSubscription := false
+		for _, s := range userSub {
+			if s.OrderId > 0 {
+				hasPaidSubscription = true
+				break
+			}
+		}
+		if hasPaidSubscription {
 			return nil, errors.Wrapf(xerr.NewErrCode(xerr.UserSubscribeExist), "user has subscription")
 		}
 	}
@@ -103,7 +111,7 @@ func (l *PurchaseLogic) Purchase(req *types.PurchaseOrderRequest) (resp *types.P
 	}
 	price := sub.UnitPrice * req.Quantity
 	// discount amount
-    amount := int64(math.Round(float64(price) * discount))
+	amount := int64(math.Round(float64(price) * discount))
 	discountAmount := price - amount
 	var coupon int64 = 0
 	// Calculate the coupon deduction