fix(subscription): 修复订阅调整逻辑,已过期订阅从当前时间计算

- 已过期订阅延长时,从当前时间开始增加天数
- 已过期订阅不允许负向调整(缩短)
- 未过期订阅保持原逻辑,从原过期时间增减

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

backend/internal/service/subscription_service.go

@@ -324,14 +324,29 @@ func (s *SubscriptionService) ExtendSubscription(ctx context.Context, subscripti
 		days = -MaxValidityDays
 	}
 
+	now := time.Now()
+	isExpired := !sub.ExpiresAt.After(now)
+
+	// 如果订阅已过期，不允许负向调整
+	if isExpired && days < 0 {
+		return nil, infraerrors.BadRequest("CANNOT_SHORTEN_EXPIRED", "cannot shorten an expired subscription")
+	}
+
 	// 计算新的过期时间
-	newExpiresAt := sub.ExpiresAt.AddDate(0, 0, days)
+	var newExpiresAt time.Time
+	if isExpired {
+		// 已过期：从当前时间开始增加天数
+		newExpiresAt = now.AddDate(0, 0, days)
+	} else {
+		// 未过期：从原过期时间增加/减少天数
+		newExpiresAt = sub.ExpiresAt.AddDate(0, 0, days)
+	}
+
 	if newExpiresAt.After(MaxExpiresAt) {
 		newExpiresAt = MaxExpiresAt
 	}
 
 	// 检查新的过期时间必须大于当前时间
-	now := time.Now()
 	if !newExpiresAt.After(now) {
 		return nil, ErrAdjustWouldExpire
 	}