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.AuditLog.Tests/Integration/Infrastructure/CombinedTelemetryDispatcher.cs

@@ -100,6 +100,7 @@ public sealed class CombinedTelemetryDispatcher : ICachedCallTelemetryForwarder
             Channel = op.Channel,
             Target = op.Target,
             SourceSite = op.SourceSite,
+            SourceNode = op.SourceNode ?? string.Empty,
             Status = op.Status,
             RetryCount = op.RetryCount,
             LastError = op.LastError ?? string.Empty,