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/SiteCallDtoMapperTests.cs

@@ -1,3 +1,4 @@
+using Google.Protobuf;
 using Google.Protobuf.WellKnownTypes;
 using ScadaLink.Communication.Grpc;
 
@@ -28,6 +29,7 @@ public class SiteCallDtoMapperTests
             Channel = "ApiOutbound",
             Target = "ERP.GetOrder",
             SourceSite = "site-melbourne",
+            SourceNode = "node-a",
             Status = "Delivered",
             RetryCount = 3,
             LastError = "transient 503",
@@ -43,6 +45,7 @@ public class SiteCallDtoMapperTests
         Assert.Equal("ApiOutbound", entity.Channel);
         Assert.Equal("ERP.GetOrder", entity.Target);
         Assert.Equal("site-melbourne", entity.SourceSite);
+        Assert.Equal("node-a", entity.SourceNode);
         Assert.Equal("Delivered", entity.Status);
         Assert.Equal(3, entity.RetryCount);
         Assert.Equal("transient 503", entity.LastError);
@@ -121,6 +124,40 @@ public class SiteCallDtoMapperTests
         Assert.Throws<ArgumentNullException>(() => SiteCallDtoMapper.FromDto(null!));
     }
 
+    [Fact]
+    public void SiteCallOperationalDto_round_trip_preserves_SourceNode()
+    {
+        // Populated SourceNode travels verbatim across the wire and through
+        // the DTO→entity mapper.
+        var dto = NewMinimalDto();
+        dto.SourceNode = "node-a";
+
+        var bytes = dto.ToByteArray();
+        var onWire = SiteCallOperationalDto.Parser.ParseFrom(bytes);
+        Assert.Equal("node-a", onWire.SourceNode);
+
+        var entity = SiteCallDtoMapper.FromDto(onWire);
+
+        Assert.Equal("node-a", entity.SourceNode);
+    }
+
+    [Fact]
+    public void SiteCallOperationalDto_round_trip_preserves_null_SourceNode()
+    {
+        // The DTO uses an empty-string-means-null convention on the wire;
+        // FromDto rehydrates that back to a true null on the entity.
+        var dto = NewMinimalDto();
+        // SourceNode left at proto default (empty string) — semantically null.
+
+        var bytes = dto.ToByteArray();
+        var onWire = SiteCallOperationalDto.Parser.ParseFrom(bytes);
+        Assert.Equal(string.Empty, onWire.SourceNode);
+
+        var entity = SiteCallDtoMapper.FromDto(onWire);
+
+        Assert.Null(entity.SourceNode);
+    }
+
     private static SiteCallOperationalDto NewMinimalDto() => new()
     {
         TrackedOperationId = Guid.NewGuid().ToString(),