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/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);