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

src/ScadaLink.Commons/Types/Notifications/SiteNotificationKpiSnapshot.cs

@@ -0,0 +1,27 @@
+namespace ScadaLink.Commons.Types.Notifications;
+
+/// <summary>
+/// Point-in-time notification-outbox metrics scoped to a single source site.
+/// The per-site counterpart of <see cref="NotificationKpiSnapshot"/>; surfaced
+/// in the per-site breakdown table on the Notification KPIs page.
+/// </summary>
+/// <param name="SourceSiteId">The site identifier these metrics are scoped to.</param>
+/// <param name="QueueDepth">Count of this site's non-terminal rows (Pending + Retrying).</param>
+/// <param name="StuckCount">
+/// Count of this site's non-terminal rows whose <c>CreatedAt</c> is older than the stuck cutoff.
+/// </param>
+/// <param name="ParkedCount">Count of this site's rows in the Parked status.</param>
+/// <param name="DeliveredLastInterval">
+/// Count of this site's Delivered rows whose <c>DeliveredAt</c> is at or after the
+/// "delivered since" timestamp.
+/// </param>
+/// <param name="OldestPendingAge">
+/// Age of this site's oldest non-terminal row, or <c>null</c> when it has none.
+/// </param>
+public record SiteNotificationKpiSnapshot(
+    string SourceSiteId,
+    int QueueDepth,
+    int StuckCount,
+    int ParkedCount,
+    int DeliveredLastInterval,
+    TimeSpan? OldestPendingAge);