feat(audit): stamp SourceNode at CentralAuditWriter + persist via AuditLogRepository

CentralAuditWriter injects INodeIdentityProvider and stamps the event before
handing to the repository. AuditLogRepository.InsertIfNotExistsAsync now
includes SourceNode in the INSERT column list. Caller-provided value wins
(supports any future direct-write callsite that already has its own node id).

tests/ScadaLink.AuditLog.Tests/Central/CentralAuditWriterTests.cs

@@ -3,6 +3,7 @@ using Microsoft.Extensions.Logging.Abstractions;
 using NSubstitute;
 using NSubstitute.ExceptionExtensions;
 using ScadaLink.AuditLog.Central;
+using ScadaLink.AuditLog.Tests.TestSupport;
 using ScadaLink.Commons.Entities.Audit;
 using ScadaLink.Commons.Interfaces.Repositories;
 using ScadaLink.Commons.Interfaces.Services;
@@ -124,4 +125,59 @@ public class CentralAuditWriterTests
         Assert.Throws<ArgumentNullException>(
             () => new CentralAuditWriter(services, null!));
     }
+
+    // ----- SourceNode stamping (Task 12) ----- //
+
+    private static (CentralAuditWriter writer, IAuditLogRepository repo) BuildWriterWithIdentity(
+        INodeIdentityProvider? nodeIdentity)
+    {
+        var repo = Substitute.For<IAuditLogRepository>();
+        var services = new ServiceCollection();
+        services.AddScoped(_ => repo);
+        var provider = services.BuildServiceProvider();
+        var writer = new CentralAuditWriter(
+            provider,
+            NullLogger<CentralAuditWriter>.Instance,
+            filter: null,
+            failureCounter: null,
+            nodeIdentity: nodeIdentity);
+        return (writer, repo);
+    }
+
+    [Fact]
+    public async Task WriteAsync_StampsSourceNodeFromProvider_WhenEventHasNone()
+    {
+        var (writer, repo) = BuildWriterWithIdentity(new FakeNodeIdentityProvider("central-a"));
+
+        await writer.WriteAsync(NewEvent());
+
+        await repo.Received(1).InsertIfNotExistsAsync(
+            Arg.Is<AuditEvent>(e => e.SourceNode == "central-a"),
+            Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task WriteAsync_PreservesCallerProvidedSourceNode()
+    {
+        var (writer, repo) = BuildWriterWithIdentity(new FakeNodeIdentityProvider("central-a"));
+        var evt = NewEvent() with { SourceNode = "central-b" };
+
+        await writer.WriteAsync(evt);
+
+        await repo.Received(1).InsertIfNotExistsAsync(
+            Arg.Is<AuditEvent>(e => e.SourceNode == "central-b"),
+            Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task WriteAsync_LeavesSourceNodeNull_WhenProviderReturnsNull()
+    {
+        var (writer, repo) = BuildWriterWithIdentity(new FakeNodeIdentityProvider(nodeName: null));
+
+        await writer.WriteAsync(NewEvent());
+
+        await repo.Received(1).InsertIfNotExistsAsync(
+            Arg.Is<AuditEvent>(e => e.SourceNode == null),
+            Arg.Any<CancellationToken>());
+    }
 }

tests/ScadaLink.ConfigurationDatabase.Tests/Repositories/AuditLogRepositoryTests.cs

@@ -50,6 +50,56 @@ public class AuditLogRepositoryTests : IClassFixture<MsSqlMigrationFixture>
         Assert.Equal(evt.EventId, loaded[0].EventId);
     }
 
+    [SkippableFact]
+    public async Task InsertIfNotExistsAsync_PersistsSourceNode()
+    {
+        Skip.IfNot(_fixture.Available, _fixture.SkipReason);
+
+        var siteId = NewSiteId();
+        await using var context = CreateContext();
+        var repo = new AuditLogRepository(context);
+
+        var evt = NewEvent(
+            siteId,
+            occurredAtUtc: new DateTime(2026, 5, 20, 10, 0, 0, DateTimeKind.Utc),
+            sourceNode: "central-a");
+        await repo.InsertIfNotExistsAsync(evt);
+
+        await using var readContext = CreateContext();
+        var loaded = await readContext.Set<AuditEvent>()
+            .Where(e => e.SourceSiteId == siteId)
+            .ToListAsync();
+
+        Assert.Single(loaded);
+        Assert.Equal("central-a", loaded[0].SourceNode);
+    }
+
+    [SkippableFact]
+    public async Task InsertIfNotExistsAsync_PersistsNullSourceNode()
+    {
+        Skip.IfNot(_fixture.Available, _fixture.SkipReason);
+
+        var siteId = NewSiteId();
+        await using var context = CreateContext();
+        var repo = new AuditLogRepository(context);
+
+        // Caller passes null SourceNode (e.g. an unconfigured node) — the
+        // column should persist as NULL, not as the empty string.
+        var evt = NewEvent(
+            siteId,
+            occurredAtUtc: new DateTime(2026, 5, 20, 10, 0, 0, DateTimeKind.Utc),
+            sourceNode: null);
+        await repo.InsertIfNotExistsAsync(evt);
+
+        await using var readContext = CreateContext();
+        var loaded = await readContext.Set<AuditEvent>()
+            .Where(e => e.SourceSiteId == siteId)
+            .ToListAsync();
+
+        Assert.Single(loaded);
+        Assert.Null(loaded[0].SourceNode);
+    }
+
     [SkippableFact]
     public async Task InsertIfNotExistsAsync_DuplicateEventId_IsNoOp_NoExceptionNoDuplicate()
     {
@@ -962,7 +1012,8 @@ public class AuditLogRepositoryTests : IClassFixture<MsSqlMigrationFixture>
         AuditStatus status = AuditStatus.Delivered,
         string? errorMessage = null,
         Guid? executionId = null,
-        Guid? parentExecutionId = null) =>
+        Guid? parentExecutionId = null,
+        string? sourceNode = null) =>
         new()
         {
             EventId = Guid.NewGuid(),
@@ -971,6 +1022,7 @@ public class AuditLogRepositoryTests : IClassFixture<MsSqlMigrationFixture>
             Kind = kind,
             Status = status,
             SourceSiteId = siteId,
+            SourceNode = sourceNode,
             ErrorMessage = errorMessage,
             ExecutionId = executionId,
             ParentExecutionId = parentExecutionId,