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.IntegrationTests/AuditLog/SiteAuditPushFlowTests.cs

@@ -146,8 +146,10 @@ public class SiteAuditPushFlowTests : TestKit
         // + Pending queue). A temp file so it survives across DI scopes.
         var dbPath = Path.Combine(Path.GetTempPath(), $"auditpush-{Guid.NewGuid():N}.db");
         var writerOptions = Options.Create(new SqliteAuditWriterOptions { DatabasePath = dbPath });
+        var nodeIdentity = Substitute.For<ScadaLink.Commons.Interfaces.Services.INodeIdentityProvider>();
+        nodeIdentity.NodeName.Returns((string?)null);
         await using var writer = new SqliteAuditWriter(
-            writerOptions, NullLogger<SqliteAuditWriter>.Instance);
+            writerOptions, NullLogger<SqliteAuditWriter>.Instance, nodeIdentity);
 
         // Real SiteCommunicationActor. RegisterCentralClient is given the relay
         // standing in for the central ClusterClient.