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/Actors/AkkaHostedService.cs

@@ -260,6 +260,36 @@ akka {{
         mgmtHolder.ActorRef = mgmtActor;
         _logger.LogInformation("ManagementActor registered with ClusterClientReceptionist");
 
+        // Notification Outbox — cluster singleton so exactly one node owns ingest,
+        // the dispatch sweep and the purge loop. Central actors run on the base
+        // "Central" role, so the singleton settings are NOT role-scoped (unlike the
+        // site singletons, which are scoped to a per-site role).
+        var outboxOptions = _serviceProvider
+            .GetRequiredService<IOptions<ScadaLink.NotificationOutbox.NotificationOutboxOptions>>().Value;
+        var outboxLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
+            .CreateLogger<ScadaLink.NotificationOutbox.NotificationOutboxActor>();
+
+        var outboxSingletonProps = ClusterSingletonManager.Props(
+            singletonProps: Props.Create(() => new ScadaLink.NotificationOutbox.NotificationOutboxActor(
+                _serviceProvider,
+                outboxOptions,
+                outboxLogger)),
+            terminationMessage: PoisonPill.Instance,
+            settings: ClusterSingletonManagerSettings.Create(_actorSystem!)
+                .WithSingletonName("notification-outbox"));
+        _actorSystem!.ActorOf(outboxSingletonProps, "notification-outbox-singleton");
+
+        var outboxProxyProps = ClusterSingletonProxy.Props(
+            singletonManagerPath: "/user/notification-outbox-singleton",
+            settings: ClusterSingletonProxySettings.Create(_actorSystem)
+                .WithSingletonName("notification-outbox"));
+        var outboxProxy = _actorSystem.ActorOf(outboxProxyProps, "notification-outbox-proxy");
+
+        // Hand the outbox proxy to the CentralCommunicationActor so forwarded
+        // NotificationSubmit messages from sites are routed to the outbox singleton.
+        centralCommActor.Tell(new RegisterNotificationOutbox(outboxProxy));
+        _logger.LogInformation("NotificationOutbox singleton created and registered with CentralCommunicationActor");
+
         _logger.LogInformation("Central actors registered. CentralCommunicationActor created.");
     }