feat(notification-outbox): register NotificationOutbox singleton in Host

Wire the Notification Outbox into the Host central role:
- Program.cs: call AddNotificationOutbox() on the central path (binds
  NotificationOutboxOptions via BindConfiguration; no explicit Configure).
- AkkaHostedService.RegisterCentralActors(): create the NotificationOutboxActor
  as a non-role-scoped central cluster singleton + proxy, then send
  RegisterNotificationOutbox(proxy) to the CentralCommunicationActor.
- appsettings.Central.json: add the ScadaLink:NotificationOutbox section with
  the NotificationOutboxOptions defaults.
- SiteServiceRegistration: remove the now-dead AddNotificationService() call -
  sites forward notifications to central rather than delivering over SMTP, and
  no site component consumes the SMTP machinery.
- Host.csproj: add the ScadaLink.NotificationOutbox project reference.
- Tests: add central outbox singleton/proxy actor-path assertions, drop the
  site OAuth2TokenService/INotificationDeliveryService resolution assertions,
  and add NotificationOutbox to the component-library IConfiguration check.

src/ScadaLink.Host/Program.cs

@@ -12,6 +12,7 @@ using ScadaLink.Host.Actors;
 using ScadaLink.Host.Health;
 using ScadaLink.InboundAPI;
 using ScadaLink.ManagementService;
+using ScadaLink.NotificationOutbox;
 using ScadaLink.NotificationService;
 using ScadaLink.Security;
 using ScadaLink.TemplateEngine;
@@ -72,6 +73,10 @@ try
         builder.Services.AddNotificationService();
 
         // Central-only components
+        // Notification Outbox: central owns SMTP delivery; the Email adapter reuses the
+        // AddNotificationService() SMTP machinery above. AddNotificationOutbox binds
+        // NotificationOutboxOptions via BindConfiguration, so no explicit Configure is needed.
+        builder.Services.AddNotificationOutbox();
         builder.Services.AddTemplateEngine();
         builder.Services.AddDeploymentManager();
         builder.Services.AddSecurity();