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

tests/ScadaLink.Commons.Tests/Types/SiteNotificationKpiSnapshotTests.cs

@@ -0,0 +1,32 @@
+using ScadaLink.Commons.Types.Notifications;
+
+namespace ScadaLink.Commons.Tests.Types;
+
+public class SiteNotificationKpiSnapshotTests
+{
+    [Fact]
+    public void Constructor_AssignsAllMembers()
+    {
+        var snapshot = new SiteNotificationKpiSnapshot(
+            SourceSiteId: "plant-a",
+            QueueDepth: 5,
+            StuckCount: 2,
+            ParkedCount: 1,
+            DeliveredLastInterval: 40,
+            OldestPendingAge: TimeSpan.FromMinutes(12));
+
+        Assert.Equal("plant-a", snapshot.SourceSiteId);
+        Assert.Equal(5, snapshot.QueueDepth);
+        Assert.Equal(2, snapshot.StuckCount);
+        Assert.Equal(1, snapshot.ParkedCount);
+        Assert.Equal(40, snapshot.DeliveredLastInterval);
+        Assert.Equal(TimeSpan.FromMinutes(12), snapshot.OldestPendingAge);
+    }
+
+    [Fact]
+    public void OldestPendingAge_IsNullableForSitesWithNoBacklog()
+    {
+        var snapshot = new SiteNotificationKpiSnapshot("plant-b", 0, 0, 0, 0, null);
+        Assert.Null(snapshot.OldestPendingAge);
+    }
+}