feat(infra): add Traefik load balancer with active node health check for central cluster failover

Add ActiveNodeHealthCheck that returns 200 only on the Akka.NET cluster
leader, enabling Traefik to route traffic to the active central node and
automatically fail over when the leader changes. Also fixes AkkaClusterHealthCheck
to resolve ActorSystem from AkkaHostedService (was always null via DI).

src/ScadaLink.Host/Program.cs

@@ -87,7 +87,8 @@ try
         // WP-12: Health checks for readiness gating
         builder.Services.AddHealthChecks()
             .AddCheck<DatabaseHealthCheck>("database")
-            .AddCheck<AkkaClusterHealthCheck>("akka-cluster");
+            .AddCheck<AkkaClusterHealthCheck>("akka-cluster")
+            .AddCheck<ActiveNodeHealthCheck>("active-node");
 
         // WP-13: Akka.NET bootstrap via hosted service
         builder.Services.AddSingleton<AkkaHostedService>();
@@ -126,6 +127,13 @@ try
             ResponseWriter = UIResponseWriter.WriteHealthCheckUIResponse
         });
 
+        // Active node endpoint — returns 200 only on the cluster leader; used by Traefik for routing
+        app.MapHealthChecks("/health/active", new HealthCheckOptions
+        {
+            Predicate = check => check.Name == "active-node",
+            ResponseWriter = UIResponseWriter.WriteHealthCheckUIResponse
+        });
+
         app.MapStaticAssets();
         app.MapCentralUI<ScadaLink.Host.Components.App>();
         app.MapInboundAPI();