feat(audit): stamp SourceNode at site SqliteAuditWriter from INodeIdentityProvider

Caller-provided SourceNode wins (preserves reconciled rows from other nodes);
otherwise the writer fills it from the local INodeIdentityProvider.NodeName.
Reads from the provider on every write — singleton lifetime means zero overhead.

tests/ScadaLink.AuditLog.Tests/AddAuditLogTests.cs

@@ -7,6 +7,7 @@ using ScadaLink.AuditLog.Central;
 using ScadaLink.AuditLog.Configuration;
 using ScadaLink.AuditLog.Site;
 using ScadaLink.AuditLog.Site.Telemetry;
+using ScadaLink.AuditLog.Tests.TestSupport;
 using ScadaLink.Commons.Interfaces.Services;
 using ScadaLink.HealthMonitoring;
 
@@ -31,6 +32,10 @@ public class AddAuditLogTests
         var services = new ServiceCollection();
         services.AddSingleton<ILoggerFactory, NullLoggerFactory>();
         services.AddSingleton(typeof(ILogger<>), typeof(NullLogger<>));
+        // INodeIdentityProvider is registered by the Host's
+        // SiteServiceRegistration in production; AddAuditLog assumes its
+        // presence so SqliteAuditWriter and CentralAuditWriter can resolve.
+        services.AddSingleton<INodeIdentityProvider>(new FakeNodeIdentityProvider());
         services.AddAuditLog(config);
         return services.BuildServiceProvider();
     }