fix: wire up health report pipeline between sites and central aggregator

Sites now send SiteHealthReport via AkkaHealthReportTransport →
SiteCommunicationActor → CentralCommunicationActor → CentralHealthAggregator.
Added IHealthReportTransport impl, ISiteIdentityProvider impl, registered
HealthReportSender on site nodes, and added SiteHealthReport handler in
CentralCommunicationActor. Health Dashboard now shows all 3 sites online.

src/ScadaLink.Host/AkkaHealthReportTransport.cs

@@ -0,0 +1,29 @@
+using Akka.Actor;
+using ScadaLink.Commons.Messages.Health;
+using ScadaLink.HealthMonitoring;
+using ScadaLink.Host.Actors;
+
+namespace ScadaLink.Host;
+
+/// <summary>
+/// Sends SiteHealthReport to the local SiteCommunicationActor via Akka ActorSelection.
+/// The SiteCommunicationActor forwards it to central.
+/// </summary>
+public class AkkaHealthReportTransport : IHealthReportTransport
+{
+    private readonly AkkaHostedService _akkaService;
+
+    public AkkaHealthReportTransport(AkkaHostedService akkaService)
+    {
+        _akkaService = akkaService;
+    }
+
+    public void Send(SiteHealthReport report)
+    {
+        var actorSystem = _akkaService.ActorSystem;
+        if (actorSystem == null) return;
+
+        var siteComm = actorSystem.ActorSelection("/user/site-communication");
+        siteComm.Tell(report, ActorRefs.NoSender);
+    }
+}

src/ScadaLink.Host/Program.cs

@@ -151,10 +151,14 @@ try
             // Shared components
             services.AddClusterInfrastructure();
             services.AddCommunication();
-            services.AddHealthMonitoring();
+            services.AddSiteHealthMonitoring();
             services.AddExternalSystemGateway();
             services.AddNotificationService();
 
+            // Health report transport: sends SiteHealthReport to SiteCommunicationActor via Akka
+            services.AddSingleton<ISiteIdentityProvider, SiteIdentityProvider>();
+            services.AddSingleton<IHealthReportTransport, AkkaHealthReportTransport>();
+
             // Site-only components — AddSiteRuntime registers SiteStorageService with SQLite path
             // and site-local repository implementations (IExternalSystemRepository, INotificationRepository)
             var siteDbPath = context.Configuration["ScadaLink:Database:SiteDbPath"] ?? "site.db";

src/ScadaLink.Host/SiteIdentityProvider.cs

@@ -0,0 +1,18 @@
+using Microsoft.Extensions.Options;
+using ScadaLink.HealthMonitoring;
+
+namespace ScadaLink.Host;
+
+/// <summary>
+/// Provides the site identity from NodeOptions configuration.
+/// </summary>
+public class SiteIdentityProvider : ISiteIdentityProvider
+{
+    public string SiteId { get; }
+
+    public SiteIdentityProvider(IOptions<NodeOptions> nodeOptions)
+    {
+        SiteId = nodeOptions.Value.SiteId
+            ?? throw new InvalidOperationException("SiteId is required for site nodes.");
+    }
+}