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,

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);
+    }
 }

tests/ScadaLink.Communication.Tests/Protos/AuditEventProtoTests.cs

@@ -25,6 +25,7 @@ public class AuditEventProtoTests
             Kind = "ApiCall",
             CorrelationId = Guid.NewGuid().ToString(),
             SourceSiteId = "site-1",
+            SourceNode = "node-a",
             SourceInstanceId = "Pump01",
             SourceScript = "OnDemand",
             Actor = "design-key",
@@ -49,6 +50,7 @@ public class AuditEventProtoTests
         Assert.Equal(original.Kind, deserialized.Kind);
         Assert.Equal(original.CorrelationId, deserialized.CorrelationId);
         Assert.Equal(original.SourceSiteId, deserialized.SourceSiteId);
+        Assert.Equal(original.SourceNode, deserialized.SourceNode);
         Assert.Equal(original.SourceInstanceId, deserialized.SourceInstanceId);
         Assert.Equal(original.SourceScript, deserialized.SourceScript);
         Assert.Equal(original.Actor, deserialized.Actor);

tests/ScadaLink.Communication.Tests/Protos/CachedTelemetryProtoTests.cs

@@ -39,6 +39,7 @@ public class CachedTelemetryProtoTests
             Channel = "ApiOutbound",
             Target = "ERP.GetOrder",
             SourceSite = "site-melbourne",
+            SourceNode = "node-a",
             Status = "Delivered",
             RetryCount = 3,
             LastError = "transient 503",
@@ -55,6 +56,7 @@ public class CachedTelemetryProtoTests
         Assert.Equal(original.Channel, deserialized.Channel);
         Assert.Equal(original.Target, deserialized.Target);
         Assert.Equal(original.SourceSite, deserialized.SourceSite);
+        Assert.Equal(original.SourceNode, deserialized.SourceNode);
         Assert.Equal(original.Status, deserialized.Status);
         Assert.Equal(original.RetryCount, deserialized.RetryCount);
         Assert.Equal(original.LastError, deserialized.LastError);

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(),