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

src/ScadaLink.Commons/Entities/Notifications/Notification.cs

@@ -25,6 +25,15 @@ public class Notification
     /// <summary>Resolved delivery targets snapshotted at delivery time, for audit.</summary>
     public string? ResolvedTargets { get; set; }
     public string SourceSiteId { get; set; }
+
+    /// <summary>
+    /// The cluster node on which the notification was emitted — `node-a` / `node-b`
+    /// for site rows (qualified by <see cref="SourceSiteId"/>), `central-a` / `central-b`
+    /// for central-originated rows. Carried from the site on the
+    /// <see cref="Commons.Messages.Notification.NotificationSubmit"/> and persisted at
+    /// central; nullable so rows submitted before the column existed don't block ingest.
+    /// </summary>
+    public string? SourceNode { get; set; }
     public string? SourceInstanceId { get; set; }
     public string? SourceScript { get; set; }