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; }
 

tests/ScadaLink.Commons.Tests/Entities/Audit/AuditEventTests.cs

@@ -116,6 +116,29 @@ public class AuditEventTests
         Assert.Null(evt.ForwardState);
     }
 
+    [Fact]
+    public void AuditEvent_carries_SourceNode_through_init()
+    {
+        // SourceNode identifies the cluster node that emitted the event (site
+        // node-a/node-b or central-a/central-b). It's an additive nullable
+        // init-only property — defaults to null when omitted, round-trips its
+        // value when set, and is preserved through `with` expressions.
+        var evtDefault = new AuditEvent
+        {
+            EventId = Guid.NewGuid(),
+            OccurredAtUtc = DateTime.UtcNow,
+            Channel = AuditChannel.ApiOutbound,
+            Kind = AuditKind.ApiCall,
+            Status = AuditStatus.Submitted,
+            PayloadTruncated = false
+        };
+        Assert.Null(evtDefault.SourceNode);
+
+        var evtStamped = evtDefault with { SourceNode = "node-a" };
+        Assert.Equal("node-a", evtStamped.SourceNode);
+        Assert.Null(evtDefault.SourceNode);
+    }
+
     [Fact]
     public void With_ProducesNewInstance_WithSingleFieldChanged()
     {