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.

src/ScadaLink.AuditLog/Site/SqliteAuditWriter.cs

@@ -42,6 +42,7 @@ public class SqliteAuditWriter : IAuditWriter, ISiteAuditQueue, IAsyncDisposable
     private readonly SqliteConnection _connection;
     private readonly SqliteAuditWriterOptions _options;
     private readonly ILogger<SqliteAuditWriter> _logger;
+    private readonly INodeIdentityProvider _nodeIdentity;
     private readonly object _writeLock = new();
     private readonly Channel<PendingAuditEvent> _writeQueue;
     private readonly Task _writerLoop;
@@ -50,13 +51,16 @@ public class SqliteAuditWriter : IAuditWriter, ISiteAuditQueue, IAsyncDisposable
     public SqliteAuditWriter(
         IOptions<SqliteAuditWriterOptions> options,
         ILogger<SqliteAuditWriter> logger,
+        INodeIdentityProvider nodeIdentity,
         string? connectionStringOverride = null)
     {
         ArgumentNullException.ThrowIfNull(options);
         ArgumentNullException.ThrowIfNull(logger);
+        ArgumentNullException.ThrowIfNull(nodeIdentity);
 
         _options = options.Value;
         _logger = logger;
+        _nodeIdentity = nodeIdentity;
 
         var connectionString = connectionStringOverride
             ?? $"Data Source={_options.DatabasePath};Cache=Shared";
@@ -325,7 +329,15 @@ public class SqliteAuditWriter : IAuditWriter, ISiteAuditQueue, IAsyncDisposable
                     pKind.Value = e.Kind.ToString();
                     pCorrelationId.Value = (object?)e.CorrelationId?.ToString() ?? DBNull.Value;
                     pSourceSiteId.Value = (object?)e.SourceSiteId ?? DBNull.Value;
-                    pSourceNode.Value = (object?)e.SourceNode ?? DBNull.Value;
+                    // SourceNode-stamping: caller-provided value wins (preserves
+                    // rows reconciled in from other nodes via the same writer);
+                    // otherwise stamp from the local INodeIdentityProvider. The
+                    // event record itself is NOT mutated — stamping is at write
+                    // time only. If the provider also returns null (unconfigured
+                    // node), the row's SourceNode stays NULL — operators see
+                    // "needs config" via the schema, not a magic fallback string.
+                    var sourceNode = e.SourceNode ?? _nodeIdentity.NodeName;
+                    pSourceNode.Value = (object?)sourceNode ?? DBNull.Value;
                     pSourceInstanceId.Value = (object?)e.SourceInstanceId ?? DBNull.Value;
                     pSourceScript.Value = (object?)e.SourceScript ?? DBNull.Value;
                     pActor.Value = (object?)e.Actor ?? DBNull.Value;