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/Integration/SyncCallEmissionEndToEndTests.cs

@@ -7,6 +7,7 @@ using ScadaLink.AuditLog.Central;
 using ScadaLink.AuditLog.Site;
 using ScadaLink.AuditLog.Site.Telemetry;
 using ScadaLink.AuditLog.Tests.Integration.Infrastructure;
+using ScadaLink.AuditLog.Tests.TestSupport;
 using ScadaLink.Commons.Entities.Audit;
 using ScadaLink.Commons.Interfaces.Repositories;
 using ScadaLink.Commons.Interfaces.Services;
@@ -91,12 +92,13 @@ public class SyncCallEmissionEndToEndTests : TestKit, IClassFixture<MsSqlMigrati
         });
 
     private static SqliteAuditWriter CreateInMemorySqliteWriter() =>
-        // The 3rd constructor argument is connectionStringOverride. A unique
+        // The 4th constructor argument is connectionStringOverride. A unique
         // shared-cache in-memory URI keeps the schema scoped to this writer
         // instance and torn down when the writer is disposed.
         new SqliteAuditWriter(
             InMemorySqliteOptions(),
             NullLogger<SqliteAuditWriter>.Instance,
+            new FakeNodeIdentityProvider(),
             connectionStringOverride: $"Data Source=file:auditlog-h-{Guid.NewGuid():N}?mode=memory&cache=shared");
 
     private static IOptions<SiteAuditTelemetryOptions> FastTelemetryOptions() =>