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

queue/logic/order/activateOrderLogic.go

@@ -206,9 +206,31 @@ func (l *ActivateOrderLogic) NewPurchase(ctx context.Context, orderInfo *order.O
 		return err
 	}
 
-	userSub, err := l.createUserSubscription(ctx, orderInfo, sub)
-	if err != nil {
-		return err
+	var userSub *user.Subscribe
+
+	// 单订阅模式下，检查用户是否已有赠送订阅（order_id=0）
+	if l.svc.Config.Subscribe.SingleModel {
+		giftSub, err := l.findGiftSubscription(ctx, orderInfo.UserId, orderInfo.SubscribeId)
+		if err == nil && giftSub != nil {
+			// 在赠送订阅上延长时间，保持 token 不变
+			userSub, err = l.extendGiftSubscription(ctx, giftSub, orderInfo, sub)
+			if err != nil {
+				logger.WithContext(ctx).Error("Extend gift subscription failed",
+					logger.Field("error", err.Error()),
+					logger.Field("gift_subscribe_id", giftSub.Id),
+				)
+				// 合并失败时回退到创建新订阅
+				userSub = nil
+			}
+		}
+	}
+
+	// 如果没有合并赠送订阅，则正常创建新订阅
+	if userSub == nil {
+		userSub, err = l.createUserSubscription(ctx, orderInfo, sub)
+		if err != nil {
+			return err
+		}
 	}
 
 	// Handle commission in separate goroutine to avoid blocking
@@ -382,6 +404,63 @@ func (l *ActivateOrderLogic) createUserSubscription(ctx context.Context, orderIn
 	return userSub, nil
 }
 
+// findGiftSubscription 查找用户指定套餐的赠送订阅（order_id=0 且状态活跃）
+// 返回找到的赠送订阅记录，如果没有则返回 nil
+func (l *ActivateOrderLogic) findGiftSubscription(ctx context.Context, userId int64, subscribeId int64) (*user.Subscribe, error) {
+	// 查询用户所有活跃订阅
+	subs, err := l.svc.UserModel.QueryUserSubscribe(ctx, userId, 0, 1)
+	if err != nil {
+		return nil, err
+	}
+	// 查找 order_id=0（赠送）且同套餐的订阅
+	for _, s := range subs {
+		if s.OrderId == 0 && s.SubscribeId == subscribeId {
+			// 通过 ID 获取完整的 Subscribe 记录
+			sub, err := l.svc.UserModel.FindOneSubscribe(ctx, s.Id)
+			if err != nil {
+				return nil, err
+			}
+			return sub, nil
+		}
+	}
+	return nil, nil
+}
+
+// extendGiftSubscription 在现有赠送订阅上延长到期时间，保持 token 不变
+// 将购买的天数叠加到赠送订阅的到期时间上，并更新 order_id 为新订单 ID
+func (l *ActivateOrderLogic) extendGiftSubscription(ctx context.Context, giftSub *user.Subscribe, orderInfo *order.Order, sub *subscribe.Subscribe) (*user.Subscribe, error) {
+	now := time.Now()
+	// 计算基准时间：取赠送订阅到期时间和当前时间的较大值
+	baseTime := giftSub.ExpireTime
+	if baseTime.Before(now) {
+		baseTime = now
+	}
+	// 在基准时间上增加购买的天数
+	newExpireTime := tool.AddTime(sub.UnitTime, orderInfo.Quantity, baseTime)
+
+	// 更新赠送订阅的信息
+	giftSub.OrderId = orderInfo.Id
+	giftSub.ExpireTime = newExpireTime
+	giftSub.Status = 1
+
+	if err := l.svc.UserModel.UpdateSubscribe(ctx, giftSub); err != nil {
+		logger.WithContext(ctx).Error("Update gift subscription failed",
+			logger.Field("error", err.Error()),
+			logger.Field("subscribe_id", giftSub.Id),
+		)
+		return nil, err
+	}
+
+	logger.WithContext(ctx).Info("Extended gift subscription successfully",
+		logger.Field("subscribe_id", giftSub.Id),
+		logger.Field("old_expire_time", baseTime),
+		logger.Field("new_expire_time", newExpireTime),
+		logger.Field("order_id", orderInfo.Id),
+	)
+
+	return giftSub, nil
+}
+
 // handleCommission processes referral commission for the referrer if applicable.
 // This runs asynchronously to avoid blocking the main order processing flow.
 func (l *ActivateOrderLogic) handleCommission(ctx context.Context, userInfo *user.User, orderInfo *order.Order) {