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.SiteRuntime.Tests/Scripts/NotifyHelperTests.cs

@@ -62,7 +62,8 @@ public class NotifyHelperTests : TestKit, IAsyncLifetime, IDisposable
         IActorRef siteCommunicationActor,
         string? sourceScript = null,
         Guid? executionId = null,
-        Guid? parentExecutionId = null)
+        Guid? parentExecutionId = null,
+        string? sourceNode = null)
     {
         return new ScriptRuntimeContext.NotifyHelper(
             _saf,
@@ -74,7 +75,8 @@ public class NotifyHelperTests : TestKit, IAsyncLifetime, IDisposable
             NullLogger.Instance,
             executionId ?? Guid.NewGuid(),
             auditWriter: null,
-            parentExecutionId: parentExecutionId);
+            parentExecutionId: parentExecutionId,
+            sourceNode: sourceNode);
     }
 
     [Fact]
@@ -197,6 +199,45 @@ public class NotifyHelperTests : TestKit, IAsyncLifetime, IDisposable
         Assert.Null(payload!.OriginParentExecutionId);
     }
 
+    [Fact]
+    public async Task Send_StampsSourceNode_OnTheNotificationSubmitPayload()
+    {
+        // SourceNode-stamping (Task 13): when the helper is wired with the
+        // local INodeIdentityProvider's NodeName, Notify.Send must stamp it
+        // onto the NotificationSubmit so it rides inside the serialized S&F
+        // payload to central, where NotificationOutboxActor persists it on
+        // the Notifications row.
+        var commProbe = CreateTestProbe();
+        var notify = CreateHelper(commProbe.Ref, sourceNode: "node-a");
+
+        var notificationId = await notify.To("Operators").Send("Pump alarm", "Pump 3 tripped");
+
+        var buffered = await _saf.GetMessageByIdAsync(notificationId);
+        Assert.NotNull(buffered);
+        var payload = JsonSerializer.Deserialize<NotificationSubmit>(buffered!.PayloadJson);
+        Assert.NotNull(payload);
+        Assert.Equal("node-a", payload!.SourceNode);
+    }
+
+    [Fact]
+    public async Task Send_NoNodeIdentity_LeavesSourceNodeNull()
+    {
+        // Hosts that don't wire INodeIdentityProvider (legacy / tests) pass
+        // null through. The NotificationSubmit payload's SourceNode stays
+        // null so the central Notifications row persists NULL rather than
+        // falling back to a placeholder.
+        var commProbe = CreateTestProbe();
+        var notify = CreateHelper(commProbe.Ref, sourceNode: null);
+
+        var notificationId = await notify.To("Operators").Send("Pump alarm", "Pump 3 tripped");
+
+        var buffered = await _saf.GetMessageByIdAsync(notificationId);
+        Assert.NotNull(buffered);
+        var payload = JsonSerializer.Deserialize<NotificationSubmit>(buffered!.PayloadJson);
+        Assert.NotNull(payload);
+        Assert.Null(payload!.SourceNode);
+    }
+
     [Fact]
     public async Task Send_WhenHelperHasNoSourceScript_LeavesSourceScriptNull()
     {