fix(notification-outbox): give KPI response a failure shape; log status-query faults

src/ScadaLink.Commons/Messages/Notification/NotificationOutboxQueries.cs

@@ -82,9 +82,13 @@ public record NotificationKpiRequest(
 
 /// <summary>
 /// Central -> Outbox UI: KPI summary for the notification outbox dashboard.
+/// On a repository fault <see cref="Success"/> is <c>false</c>, <see cref="ErrorMessage"/>
+/// carries the cause, and the KPI fields are zeroed/<c>null</c>.
 /// </summary>
 public record NotificationKpiResponse(
     string CorrelationId,
+    bool Success,
+    string? ErrorMessage,
     int QueueDepth,
     int StuckCount,
     int ParkedCount,