feat(notification-outbox): per-site KPI snapshot type + repository contract

src/ScadaLink.Commons/Interfaces/Repositories/INotificationOutboxRepository.cs

@@ -62,6 +62,15 @@ public interface INotificationOutboxRepository
     Task<NotificationKpiSnapshot> ComputeKpisAsync(
         DateTimeOffset stuckCutoff, DateTimeOffset deliveredSince, CancellationToken cancellationToken = default);
 
+    /// <summary>
+    /// Computes a point-in-time <see cref="SiteNotificationKpiSnapshot"/> per source site.
+    /// Sites with no notification rows at all are omitted. The stuck and delivered cutoffs
+    /// are supplied by the caller; the current time used for <c>OldestPendingAge</c> is
+    /// captured inside the method.
+    /// </summary>
+    Task<IReadOnlyList<SiteNotificationKpiSnapshot>> ComputePerSiteKpisAsync(
+        DateTimeOffset stuckCutoff, DateTimeOffset deliveredSince, CancellationToken cancellationToken = default);
+
     /// <summary>
     /// Persists pending changes tracked on the underlying context. Use this when staging
     /// multiple changes for a single commit; the individual mutating methods on this