feat(host): add NodeName to NodeOptions + INodeIdentityProvider

- NodeName: semantic role-within-cluster identifier (node-a/node-b on sites,
  central-a/central-b on central). Bound from ScadaLink:Node:NodeName.
- INodeIdentityProvider exposes the trimmed name (null if unconfigured) so
  downstream audit writers can stamp the new SourceNode column.

src/ScadaLink.Host/SiteServiceRegistration.cs

@@ -1,6 +1,7 @@
 using ScadaLink.AuditLog;
 using ScadaLink.ClusterInfrastructure;
 using ScadaLink.Communication;
+using ScadaLink.Commons.Interfaces.Services;
 using ScadaLink.DataConnectionLayer;
 using ScadaLink.ExternalSystemGateway;
 using ScadaLink.HealthMonitoring;
@@ -96,5 +97,10 @@ public static class SiteServiceRegistration
         services.Configure<HealthMonitoringOptions>(config.GetSection("ScadaLink:HealthMonitoring"));
         services.Configure<NotificationOptions>(config.GetSection("ScadaLink:Notification"));
         services.Configure<LoggingOptions>(config.GetSection("ScadaLink:Logging"));
+
+        // Audit Log (#23) — exposes ScadaLink:Node:NodeName to downstream audit
+        // writers so they can stamp the SourceNode column. Registered here in
+        // shared bootstrap because every node (central + site) needs it.
+        services.AddSingleton<INodeIdentityProvider, NodeIdentityProvider>();
     }
 }