feat(notification-outbox): per-site KPI request/response message contracts

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

@@ -1,3 +1,5 @@
+using ScadaLink.Commons.Types.Notifications;
+
 namespace ScadaLink.Commons.Messages.Notification;
 
 /// <summary>
@@ -94,3 +96,20 @@ public record NotificationKpiResponse(
     int ParkedCount,
     int DeliveredLastInterval,
     TimeSpan? OldestPendingAge);
+
+/// <summary>
+/// Outbox UI -> Central: request for the per-source-site notification outbox KPI breakdown.
+/// </summary>
+public record PerSiteNotificationKpiRequest(
+    string CorrelationId);
+
+/// <summary>
+/// Central -> Outbox UI: per-site KPI breakdown for the Notification KPIs page.
+/// On a repository fault <see cref="Success"/> is <c>false</c>, <see cref="ErrorMessage"/>
+/// carries the cause, and <see cref="Sites"/> is empty.
+/// </summary>
+public record PerSiteNotificationKpiResponse(
+    string CorrelationId,
+    bool Success,
+    string? ErrorMessage,
+    IReadOnlyList<SiteNotificationKpiSnapshot> Sites);