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/AuditEventDtoMapperTests.cs

@@ -34,6 +34,7 @@ public class AuditEventDtoMapperTests
             ExecutionId = executionId,
             ParentExecutionId = parentExecutionId,
             SourceSiteId = "site-1",
+            SourceNode = "node-a",
             SourceInstanceId = "Pump01",
             SourceScript = "OnDemand",
             Actor = "design-key",
@@ -61,6 +62,7 @@ public class AuditEventDtoMapperTests
         Assert.Equal(original.ExecutionId, roundTripped.ExecutionId);
         Assert.Equal(original.ParentExecutionId, roundTripped.ParentExecutionId);
         Assert.Equal(original.SourceSiteId, roundTripped.SourceSiteId);
+        Assert.Equal(original.SourceNode, roundTripped.SourceNode);
         Assert.Equal(original.SourceInstanceId, roundTripped.SourceInstanceId);
         Assert.Equal(original.SourceScript, roundTripped.SourceScript);
         Assert.Equal(original.Actor, roundTripped.Actor);
@@ -99,6 +101,7 @@ public class AuditEventDtoMapperTests
         Assert.Equal(string.Empty, dto.ExecutionId);
         Assert.Equal(string.Empty, dto.ParentExecutionId);
         Assert.Equal(string.Empty, dto.SourceSiteId);
+        Assert.Equal(string.Empty, dto.SourceNode);
         Assert.Equal(string.Empty, dto.SourceInstanceId);
         Assert.Equal(string.Empty, dto.SourceScript);
         Assert.Equal(string.Empty, dto.Actor);
@@ -124,6 +127,7 @@ public class AuditEventDtoMapperTests
             ExecutionId = string.Empty,
             ParentExecutionId = string.Empty,
             SourceSiteId = string.Empty,
+            SourceNode = string.Empty,
             SourceInstanceId = string.Empty,
             SourceScript = string.Empty,
             Actor = string.Empty,
@@ -141,6 +145,7 @@ public class AuditEventDtoMapperTests
         Assert.Null(evt.ExecutionId);
         Assert.Null(evt.ParentExecutionId);
         Assert.Null(evt.SourceSiteId);
+        Assert.Null(evt.SourceNode);
         Assert.Null(evt.SourceInstanceId);
         Assert.Null(evt.SourceScript);
         Assert.Null(evt.Actor);
@@ -232,4 +237,52 @@ public class AuditEventDtoMapperTests
         Assert.Equal("ApiCallCached", dto.Kind);
         Assert.Equal("Parked", dto.Status);
     }
+
+    [Fact]
+    public void AuditEventDto_round_trip_preserves_SourceNode()
+    {
+        var evt = new AuditEvent
+        {
+            EventId = Guid.NewGuid(),
+            OccurredAtUtc = DateTime.UtcNow,
+            Channel = AuditChannel.ApiOutbound,
+            Kind = AuditKind.ApiCall,
+            Status = AuditStatus.Delivered,
+            SourceNode = "node-a"
+        };
+
+        var dto = AuditEventDtoMapper.ToDto(evt);
+
+        // Wire form: empty-string-means-null convention; populated value
+        // travels verbatim.
+        Assert.Equal("node-a", dto.SourceNode);
+
+        var roundTripped = AuditEventDtoMapper.FromDto(dto);
+
+        Assert.Equal("node-a", roundTripped.SourceNode);
+    }
+
+    [Fact]
+    public void AuditEventDto_round_trip_preserves_null_SourceNode()
+    {
+        var evt = new AuditEvent
+        {
+            EventId = Guid.NewGuid(),
+            OccurredAtUtc = DateTime.UtcNow,
+            Channel = AuditChannel.ApiOutbound,
+            Kind = AuditKind.ApiCall,
+            Status = AuditStatus.Delivered,
+            SourceNode = null
+        };
+
+        var dto = AuditEventDtoMapper.ToDto(evt);
+
+        // ToDto collapses null → empty on the wire…
+        Assert.Equal(string.Empty, dto.SourceNode);
+
+        var roundTripped = AuditEventDtoMapper.FromDto(dto);
+
+        // …and FromDto rehydrates empty → null.
+        Assert.Null(roundTripped.SourceNode);
+    }
 }