feat(notif-outbox): carry + persist SourceNode end-to-end via NotificationSubmit

Site: inject INodeIdentityProvider where NotificationSubmit is built; stamp
SourceNode = NodeName at construction.

Central: NotificationOutboxActor.HandleSubmit copies submit.SourceNode onto
the Notification row; the repository INSERT persists it (EF tracked-entity
insert flows it through automatically; raw-SQL extension if not).

After this commit, every Notifications row carries the originating site
node-a/node-b in SourceNode. Existing notifications submitted pre-feature
remain NULL.

tests/ScadaLink.NotificationOutbox.Tests/NotificationOutboxActorIngestTests.cs

@@ -42,7 +42,8 @@ public class NotificationOutboxActorIngestTests : TestKit
     private static NotificationSubmit MakeSubmit(
         string? notificationId = null,
         Guid? originExecutionId = null,
-        Guid? originParentExecutionId = null)
+        Guid? originParentExecutionId = null,
+        string? sourceNode = null)
     {
         return new NotificationSubmit(
             NotificationId: notificationId ?? Guid.NewGuid().ToString(),
@@ -54,7 +55,8 @@ public class NotificationOutboxActorIngestTests : TestKit
             SourceScript: "AlarmScript",
             SiteEnqueuedAt: new DateTimeOffset(2026, 5, 19, 8, 30, 0, TimeSpan.Zero),
             OriginExecutionId: originExecutionId,
-            OriginParentExecutionId: originParentExecutionId);
+            OriginParentExecutionId: originParentExecutionId,
+            SourceNode: sourceNode);
     }
 
     [Fact]
@@ -192,4 +194,43 @@ public class NotificationOutboxActorIngestTests : TestKit
         Assert.NotNull(ack.Error);
         Assert.Contains("database unavailable", ack.Error);
     }
+
+    [Fact]
+    public void NotificationSubmit_CopiesSourceNode_OntoPersistedNotification()
+    {
+        // SourceNode-stamping (Task 13): the originating site's node name (node-a/node-b)
+        // rides on the NotificationSubmit and must be persisted on the Notification row so
+        // central observers (KPIs, audit drill-ins, ops dashboards) can see which node
+        // emitted the notification.
+        _repository.InsertIfNotExistsAsync(Arg.Any<Notification>(), Arg.Any<CancellationToken>())
+            .Returns(true);
+        var submit = MakeSubmit(sourceNode: "node-a");
+        var actor = CreateActor();
+
+        actor.Tell(submit, TestActor);
+
+        ExpectMsg<NotificationSubmitAck>();
+        _repository.Received(1).InsertIfNotExistsAsync(
+            Arg.Is<Notification>(n => n.SourceNode == "node-a"),
+            Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public void NotificationSubmit_NullSourceNode_PersistsNull()
+    {
+        // Submissions from a host that didn't wire INodeIdentityProvider, or from
+        // pre-SourceNode-stamping clients, carry null SourceNode — the central row must
+        // persist NULL rather than fall back to a placeholder.
+        _repository.InsertIfNotExistsAsync(Arg.Any<Notification>(), Arg.Any<CancellationToken>())
+            .Returns(true);
+        var submit = MakeSubmit(sourceNode: null);
+        var actor = CreateActor();
+
+        actor.Tell(submit, TestActor);
+
+        ExpectMsg<NotificationSubmitAck>();
+        _repository.Received(1).InsertIfNotExistsAsync(
+            Arg.Is<Notification>(n => n.SourceNode == null),
+            Arg.Any<CancellationToken>());
+    }
 }