feat(sitecall-audit): add SourceNode to SiteCallOperational + SiteCall entity

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

@@ -30,6 +30,15 @@ public sealed record SiteCall
     /// <summary>Site id that submitted the cached call.</summary>
     public required string SourceSite { get; init; }
 
+    /// <summary>
+    /// The cluster node on which the cached call was emitted — <c>node-a</c> /
+    /// <c>node-b</c> for site rows (qualified by <see cref="SourceSite"/>),
+    /// <c>central-a</c> / <c>central-b</c> for central-originated rows. Stamped
+    /// by the emitting 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>
     /// Lifecycle status — string form of
     /// <see cref="ScadaLink.Commons.Types.Enums.AuditStatus"/>. Monotonic: later rank