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);
+    }
+}