feat(comm): add source_node field to AuditEventDto + SiteCallOperationalDto proto

- AuditEventDto field 22, SiteCallOperationalDto field 12. Both follow the
  existing empty-string-means-null convention.
- Mappers carry SourceNode end-to-end; round-trip tests cover both populated
  and null cases.

tests/ScadaLink.Communication.Tests/Protos/AuditEventProtoTests.cs

@@ -25,6 +25,7 @@ public class AuditEventProtoTests
             Kind = "ApiCall",
             CorrelationId = Guid.NewGuid().ToString(),
             SourceSiteId = "site-1",
+            SourceNode = "node-a",
             SourceInstanceId = "Pump01",
             SourceScript = "OnDemand",
             Actor = "design-key",
@@ -49,6 +50,7 @@ public class AuditEventProtoTests
         Assert.Equal(original.Kind, deserialized.Kind);
         Assert.Equal(original.CorrelationId, deserialized.CorrelationId);
         Assert.Equal(original.SourceSiteId, deserialized.SourceSiteId);
+        Assert.Equal(original.SourceNode, deserialized.SourceNode);
         Assert.Equal(original.SourceInstanceId, deserialized.SourceInstanceId);
         Assert.Equal(original.SourceScript, deserialized.SourceScript);
         Assert.Equal(original.Actor, deserialized.Actor);

tests/ScadaLink.Communication.Tests/Protos/CachedTelemetryProtoTests.cs

@@ -39,6 +39,7 @@ public class CachedTelemetryProtoTests
             Channel = "ApiOutbound",
             Target = "ERP.GetOrder",
             SourceSite = "site-melbourne",
+            SourceNode = "node-a",
             Status = "Delivered",
             RetryCount = 3,
             LastError = "transient 503",
@@ -55,6 +56,7 @@ public class CachedTelemetryProtoTests
         Assert.Equal(original.Channel, deserialized.Channel);
         Assert.Equal(original.Target, deserialized.Target);
         Assert.Equal(original.SourceSite, deserialized.SourceSite);
+        Assert.Equal(original.SourceNode, deserialized.SourceNode);
         Assert.Equal(original.Status, deserialized.Status);
         Assert.Equal(original.RetryCount, deserialized.RetryCount);
         Assert.Equal(original.LastError, deserialized.LastError);