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/SiteServiceRegistration.cs

@@ -25,7 +25,12 @@ public static class SiteServiceRegistration
         services.AddCommunication();
         services.AddSiteHealthMonitoring();
         services.AddExternalSystemGateway();
-        services.AddNotificationService();
+        // AddNotificationService() is intentionally NOT registered on the site path.
+        // Sites no longer deliver notifications over SMTP — a buffered notification is
+        // forwarded to the central cluster (via NotificationForwarder / SiteCommunicationActor),
+        // and central owns SMTP delivery through the Notification Outbox. The SMTP machinery
+        // (OAuth2TokenService, ISmtpClientWrapper, INotificationDeliveryService) has no
+        // consumer on a site node.
 
         // Health report transport: sends SiteHealthReport to SiteCommunicationActor via Akka
         services.AddSingleton<ISiteIdentityProvider, SiteIdentityProvider>();