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

tests/ScadaLink.Commons.Tests/Messages/NotificationMessagesTests.cs

@@ -126,6 +126,32 @@ public class NotificationMessagesTests
         Assert.Equal(parentExecutionId, roundTripped!.OriginParentExecutionId);
     }
 
+    [Fact]
+    public void NotificationSubmit_carries_SourceNode()
+    {
+        // SourceNode is an additive trailing member — old call sites and old
+        // serialized payloads leave it null. When supplied it round-trips
+        // through both construction and JSON (the buffered S&F payload IS a
+        // serialized NotificationSubmit).
+        var defaulted = new NotificationSubmit(
+            "notif-9", "Operators", "Subject", "Body",
+            "site-01", "inst-1", "OnAlarm", DateTimeOffset.UtcNow);
+        Assert.Null(defaulted.SourceNode);
+
+        var stamped = new NotificationSubmit(
+            "notif-10", "Operators", "Subject", "Body",
+            "site-01", "inst-1", "OnAlarm", DateTimeOffset.UtcNow,
+            OriginExecutionId: null,
+            OriginParentExecutionId: null,
+            SourceNode: "node-a");
+        Assert.Equal("node-a", stamped.SourceNode);
+
+        var json = System.Text.Json.JsonSerializer.Serialize(stamped);
+        var roundTripped = System.Text.Json.JsonSerializer.Deserialize<NotificationSubmit>(json);
+        Assert.NotNull(roundTripped);
+        Assert.Equal("node-a", roundTripped!.SourceNode);
+    }
+
     [Fact]
     public void NotificationSubmit_ValueEquality_EqualWhenAllFieldsMatch()
     {