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/Site/SqliteAuditWriterBacklogStatsTests.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
 using ScadaLink.AuditLog.Site;
+using ScadaLink.AuditLog.Tests.TestSupport;
 using ScadaLink.Commons.Entities.Audit;
 using ScadaLink.Commons.Types.Enums;
 
@@ -39,7 +40,8 @@ public class SqliteAuditWriterBacklogStatsTests : IDisposable
         var options = new SqliteAuditWriterOptions { DatabasePath = _dbPath };
         return new SqliteAuditWriter(
             Options.Create(options),
-            NullLogger<SqliteAuditWriter>.Instance);
+            NullLogger<SqliteAuditWriter>.Instance,
+            new FakeNodeIdentityProvider());
     }
 
     private static AuditEvent NewEvent(DateTime? occurredAtUtc = null) => new()