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/Health/AkkaClusterHealthCheck.cs

@@ -1,6 +1,6 @@
-using Akka.Actor;
 using Akka.Cluster;
 using Microsoft.Extensions.Diagnostics.HealthChecks;
+using ScadaLink.Host.Actors;
 
 namespace ScadaLink.Host.Health;
 
@@ -10,21 +10,22 @@ namespace ScadaLink.Host.Health;
 /// </summary>
 public class AkkaClusterHealthCheck : IHealthCheck
 {
-    private readonly ActorSystem? _system;
+    private readonly AkkaHostedService _akkaService;
 
-    public AkkaClusterHealthCheck(ActorSystem? system = null)
+    public AkkaClusterHealthCheck(AkkaHostedService akkaService)
     {
-        _system = system;
+        _akkaService = akkaService;
     }
 
     public Task<HealthCheckResult> CheckHealthAsync(
         HealthCheckContext context,
         CancellationToken cancellationToken = default)
     {
-        if (_system == null)
+        var system = _akkaService.ActorSystem;
+        if (system == null)
             return Task.FromResult(HealthCheckResult.Degraded("ActorSystem not yet available."));
 
-        var cluster = Cluster.Get(_system);
+        var cluster = Cluster.Get(system);
         var status = cluster.SelfMember.Status;
 
         var result = status switch