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/CompositionRootTests.cs

@@ -353,7 +353,6 @@ public class SiteCompositionRootTests : IDisposable
         new object[] { typeof(ReplicationService) },
         new object[] { typeof(ISiteEventLogger) },
         new object[] { typeof(IEventLogQueryService) },
-        new object[] { typeof(OAuth2TokenService) },
         new object[] { typeof(ISiteIdentityProvider) },
         new object[] { typeof(IHealthReportTransport) },
     };
@@ -377,8 +376,6 @@ public class SiteCompositionRootTests : IDisposable
         new object[] { typeof(IExternalSystemClient) },
         new object[] { typeof(DatabaseGateway) },
         new object[] { typeof(IDatabaseGateway) },
-        new object[] { typeof(NotificationDeliveryService) },
-        new object[] { typeof(INotificationDeliveryService) },
     };
 
     // --- Implementation type assertions ---