feat(sub): expose last subscription fetch time (#6217)

* feat(sub): expose last subscription fetch time

Record successful GET subscription fetches per client and surface the timestamp in the client API and UI.

* fix(frontend): include last subscription fetch in client traffic

* Update sub_fetch_test.go

---------

Co-authored-by: Hermes Agent <hermes-agent@localhost>

internal/sub/service.go

@@ -273,6 +273,16 @@ func (s *SubService) matchingClients(inbound *model.Inbound, subId string) []mod
 	return out
 }
 
+// RecordSubscriptionFetch records a successful subscription response for all clients sharing subId.
+func (s *SubService) RecordSubscriptionFetch(subId string) error {
+	if strings.TrimSpace(subId) == "" {
+		return nil
+	}
+	return database.GetDB().Model(&xray.ClientTraffic{}).
+		Where("email IN (SELECT email FROM clients WHERE sub_id = ?)", subId).
+		Update("last_sub_fetch", time.Now().UnixMilli()).Error
+}
+
 // GetSubs retrieves subscription links for a given subscription ID and host.
 func (s *SubService) GetSubs(subId string, host string) ([]string, []string, int64, xray.ClientTraffic, error) {
 	return s.ForRequest(host).getSubs(subId)