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/AuditEventDtoMapper.cs

@@ -50,6 +50,7 @@ public static class AuditEventDtoMapper
             ExecutionId = evt.ExecutionId?.ToString() ?? string.Empty,
             ParentExecutionId = evt.ParentExecutionId?.ToString() ?? string.Empty,
             SourceSiteId = evt.SourceSiteId ?? string.Empty,
+            SourceNode = evt.SourceNode ?? string.Empty,
             SourceInstanceId = evt.SourceInstanceId ?? string.Empty,
             SourceScript = evt.SourceScript ?? string.Empty,
             Actor = evt.Actor ?? string.Empty,
@@ -97,6 +98,7 @@ public static class AuditEventDtoMapper
             ExecutionId = NullIfEmpty(dto.ExecutionId) is { } eid ? Guid.Parse(eid) : null,
             ParentExecutionId = NullIfEmpty(dto.ParentExecutionId) is { } pid ? Guid.Parse(pid) : null,
             SourceSiteId = NullIfEmpty(dto.SourceSiteId),
+            SourceNode = NullIfEmpty(dto.SourceNode),
             SourceInstanceId = NullIfEmpty(dto.SourceInstanceId),
             SourceScript = NullIfEmpty(dto.SourceScript),
             Actor = NullIfEmpty(dto.Actor),

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,