feat(subscription): 支持调整订阅时长（延长/缩短）

- 将"延长订阅"功能改为"调整订阅"，支持正数延长、负数缩短
- 后端验证：调整天数范围 -36500 到 36500，缩短后剩余天数必须 > 0
- 前端同步更新界面文案和验证逻辑

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/service/subscription_service.go

@@ -27,6 +27,7 @@ var (
 	ErrWeeklyLimitExceeded       = infraerrors.TooManyRequests("WEEKLY_LIMIT_EXCEEDED", "weekly usage limit exceeded")
 	ErrMonthlyLimitExceeded      = infraerrors.TooManyRequests("MONTHLY_LIMIT_EXCEEDED", "monthly usage limit exceeded")
 	ErrSubscriptionNilInput      = infraerrors.BadRequest("SUBSCRIPTION_NIL_INPUT", "subscription input cannot be nil")
+	ErrAdjustWouldExpire         = infraerrors.BadRequest("ADJUST_WOULD_EXPIRE", "adjustment would result in expired subscription (remaining days must be > 0)")
 )
 
 // SubscriptionService 订阅服务
@@ -308,17 +309,20 @@ func (s *SubscriptionService) RevokeSubscription(ctx context.Context, subscripti
 	return nil
 }
 
-// ExtendSubscription 延长订阅
+// ExtendSubscription 调整订阅时长（正数延长，负数缩短）
 func (s *SubscriptionService) ExtendSubscription(ctx context.Context, subscriptionID int64, days int) (*UserSubscription, error) {
 	sub, err := s.userSubRepo.GetByID(ctx, subscriptionID)
 	if err != nil {
 		return nil, ErrSubscriptionNotFound
 	}
 
-	// 限制延长天数
+	// 限制调整天数范围
 	if days > MaxValidityDays {
 		days = MaxValidityDays
 	}
+	if days < -MaxValidityDays {
+		days = -MaxValidityDays
+	}
 
 	// 计算新的过期时间
 	newExpiresAt := sub.ExpiresAt.AddDate(0, 0, days)
@@ -326,6 +330,14 @@ func (s *SubscriptionService) ExtendSubscription(ctx context.Context, subscripti
 		newExpiresAt = MaxExpiresAt
 	}
 
+	// 如果是缩短（负数），检查新的过期时间必须大于当前时间
+	if days < 0 {
+		now := time.Now()
+		if !newExpiresAt.After(now) {
+			return nil, ErrAdjustWouldExpire
+		}
+	}
+
 	if err := s.userSubRepo.ExtendExpiry(ctx, subscriptionID, newExpiresAt); err != nil {
 		return nil, err
 	}