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/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.");
+    }
+}