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/ExecutionIdCorrelationTests.cs

@@ -9,6 +9,7 @@ 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.Interfaces.Repositories;
 using ScadaLink.Commons.Interfaces.Services;
 using ScadaLink.Commons.Types.Audit;
@@ -111,6 +112,7 @@ public class ExecutionIdCorrelationTests : TestKit, IClassFixture<MsSqlMigration
                 ChannelCapacity = 1024,
             }),
             NullLogger<SqliteAuditWriter>.Instance,
+            new FakeNodeIdentityProvider(),
             connectionStringOverride:
                 $"Data Source=file:auditlog-execid-{Guid.NewGuid():N}?mode=memory&cache=shared");