feat(notif-outbox): add SourceNode to Notification entity + NotificationSubmit

tests/ScadaLink.Commons.Tests/Entities/NotificationEntityTests.cs

@@ -51,6 +51,20 @@ public class NotificationEntityTests
         Assert.Equal(parentExecutionId, n.OriginParentExecutionId);
     }
 
+    [Fact]
+    public void SourceNode_DefaultsToNull_AndIsSettable()
+    {
+        // SourceNode identifies the cluster node that emitted the notification
+        // (site node-a/node-b or central-a/central-b). Additive nullable
+        // property — defaults to null on rows submitted before the column
+        // existed, and round-trips its value when set.
+        var n = new Notification("id-1", NotificationType.Email, "ops-team", "subj", "body", "SiteA");
+        Assert.Null(n.SourceNode);
+
+        n.SourceNode = "node-a";
+        Assert.Equal("node-a", n.SourceNode);
+    }
+
     [Fact]
     public void Constructor_NullArguments_Throw()
     {