feat(audit): add SourceNode property to AuditEvent record

src/ScadaLink.Commons/Entities/Audit/AuditEvent.cs

@@ -43,6 +43,15 @@ public sealed record AuditEvent
     /// <summary>Site id where the action originated; null for central-direct events.</summary>
     public string? SourceSiteId { get; init; }
 
+    /// <summary>
+    /// The cluster node on which the event was emitted — `node-a` / `node-b` for
+    /// site rows (qualified by <see cref="SourceSiteId"/>), `central-a` / `central-b`
+    /// for central-originated rows. Stamped by the writing node from
+    /// <c>INodeIdentityProvider</c>; nullable so reconciled rows from a node that
+    /// has since been retired don't block ingest.
+    /// </summary>
+    public string? SourceNode { get; init; }
+
     /// <summary>Instance id where the action originated, when applicable.</summary>
     public string? SourceInstanceId { get; init; }