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/Infrastructure/CombinedTelemetryHarness.cs

@@ -10,6 +10,7 @@ using ScadaLink.Commons.Interfaces;
 using ScadaLink.Commons.Interfaces.Repositories;
 using ScadaLink.Commons.Interfaces.Services;
 using ScadaLink.Commons.Messages.Integration;
+using ScadaLink.AuditLog.Tests.TestSupport;
 using ScadaLink.ConfigurationDatabase;
 using ScadaLink.ConfigurationDatabase.Repositories;
 using ScadaLink.ConfigurationDatabase.Tests.Migrations;
@@ -78,6 +79,7 @@ public sealed class CombinedTelemetryHarness : IAsyncDisposable
                 ChannelCapacity = 1024,
             }),
             NullLogger<SqliteAuditWriter>.Instance,
+            new FakeNodeIdentityProvider(),
             connectionStringOverride:
                 $"Data Source=file:cachedcall-g-{Guid.NewGuid():N}?mode=memory&cache=shared");