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/SqliteAuditWriterSchemaTests.cs

@@ -2,6 +2,7 @@ using Microsoft.Data.Sqlite;
 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;
 
@@ -31,6 +32,7 @@ public class SqliteAuditWriterSchemaTests
         var writer = new SqliteAuditWriter(
             Options.Create(options),
             NullLogger<SqliteAuditWriter>.Instance,
+            new FakeNodeIdentityProvider(),
             connectionStringOverride: $"Data Source={dataSource};Cache=Shared");
         return (writer, dataSource);
     }
@@ -200,6 +202,7 @@ public class SqliteAuditWriterSchemaTests
         return new SqliteAuditWriter(
             Options.Create(options),
             NullLogger<SqliteAuditWriter>.Instance,
+            new FakeNodeIdentityProvider(),
             connectionStringOverride: $"Data Source={dataSource};Cache=Shared");
     }