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.");
     }
 

src/ScadaLink.Host/Program.cs

@@ -12,6 +12,7 @@ using ScadaLink.Host.Actors;
 using ScadaLink.Host.Health;
 using ScadaLink.InboundAPI;
 using ScadaLink.ManagementService;
+using ScadaLink.NotificationOutbox;
 using ScadaLink.NotificationService;
 using ScadaLink.Security;
 using ScadaLink.TemplateEngine;
@@ -72,6 +73,10 @@ try
         builder.Services.AddNotificationService();
 
         // Central-only components
+        // Notification Outbox: central owns SMTP delivery; the Email adapter reuses the
+        // AddNotificationService() SMTP machinery above. AddNotificationOutbox binds
+        // NotificationOutboxOptions via BindConfiguration, so no explicit Configure is needed.
+        builder.Services.AddNotificationOutbox();
         builder.Services.AddTemplateEngine();
         builder.Services.AddDeploymentManager();
         builder.Services.AddSecurity();

src/ScadaLink.Host/ScadaLink.Host.csproj

@@ -37,6 +37,7 @@
     <ProjectReference Include="../ScadaLink.StoreAndForward/ScadaLink.StoreAndForward.csproj" />
     <ProjectReference Include="../ScadaLink.ExternalSystemGateway/ScadaLink.ExternalSystemGateway.csproj" />
     <ProjectReference Include="../ScadaLink.NotificationService/ScadaLink.NotificationService.csproj" />
+    <ProjectReference Include="../ScadaLink.NotificationOutbox/ScadaLink.NotificationOutbox.csproj" />
     <ProjectReference Include="../ScadaLink.CentralUI/ScadaLink.CentralUI.csproj" />
     <ProjectReference Include="../ScadaLink.Security/ScadaLink.Security.csproj" />
     <ProjectReference Include="../ScadaLink.HealthMonitoring/ScadaLink.HealthMonitoring.csproj" />

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>();

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"
     }

tests/ScadaLink.Host.Tests/ActorPathTests.cs

@@ -97,6 +97,14 @@ public class CentralActorPathTests : IAsyncLifetime
     public async Task CentralActors_Management_Exists()
         => await AssertActorExists("/user/management");
 
+    [Fact]
+    public async Task CentralActors_NotificationOutboxSingleton_Exists()
+        => await AssertActorExists("/user/notification-outbox-singleton");
+
+    [Fact]
+    public async Task CentralActors_NotificationOutboxProxy_Exists()
+        => await AssertActorExists("/user/notification-outbox-proxy");
+
     private async Task AssertActorExists(string path)
     {
         Assert.NotNull(_actorSystem);

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 ---

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,