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/appsettings.Central.json

@@ -52,6 +52,14 @@
       "AuthMode": "None",
       "FromAddress": "scada-notifications@company.com"
     },
+    "NotificationOutbox": {
+      "DispatchInterval": "00:00:10",
+      "DispatchBatchSize": 100,
+      "StuckAgeThreshold": "00:10:00",
+      "TerminalRetention": "365.00:00:00",
+      "PurgeInterval": "1.00:00:00",
+      "DeliveredKpiWindow": "00:01:00"
+    },
     "Logging": {
       "MinimumLevel": "Information"
     }