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.

src/ScadaLink.Communication/Grpc/SiteCallDtoMapper.cs

@@ -55,6 +55,7 @@ public static class SiteCallDtoMapper
             Channel = dto.Channel,
             Target = dto.Target,
             SourceSite = dto.SourceSite,
+            SourceNode = string.IsNullOrEmpty(dto.SourceNode) ? null : dto.SourceNode,
             Status = dto.Status,
             RetryCount = dto.RetryCount,
             LastError = string.IsNullOrEmpty(dto.LastError) ? null : dto.LastError,