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/Payload/FilterIntegrationTests.cs

@@ -11,6 +11,7 @@ using ScadaLink.AuditLog.Central;
 using ScadaLink.AuditLog.Configuration;
 using ScadaLink.AuditLog.Payload;
 using ScadaLink.AuditLog.Site;
+using ScadaLink.AuditLog.Tests.TestSupport;
 using ScadaLink.Commons.Entities.Audit;
 using ScadaLink.Commons.Interfaces.Repositories;
 using ScadaLink.Commons.Interfaces.Services;
@@ -89,6 +90,7 @@ public class FilterIntegrationTests
         var sqliteWriter = new SqliteAuditWriter(
             Microsoft.Extensions.Options.Options.Create(new SqliteAuditWriterOptions { DatabasePath = dataSource }),
             NullLogger<SqliteAuditWriter>.Instance,
+            new FakeNodeIdentityProvider(),
             connectionStringOverride: $"Data Source={dataSource};Cache=Shared");
         await using var _disposeSqlite = sqliteWriter;