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.

tests/ScadaLink.Host.Tests/OptionsTests.cs

@@ -21,6 +21,7 @@ public class OptionsTests
             typeof(HealthMonitoring.ServiceCollectionExtensions).Assembly,
             typeof(ExternalSystemGateway.ServiceCollectionExtensions).Assembly,
             typeof(NotificationService.ServiceCollectionExtensions).Assembly,
+            typeof(NotificationOutbox.ServiceCollectionExtensions).Assembly,
             typeof(TemplateEngine.ServiceCollectionExtensions).Assembly,
             typeof(DeploymentManager.ServiceCollectionExtensions).Assembly,
             typeof(Security.ServiceCollectionExtensions).Assembly,