feat: replace ActorSelection with ClusterClient for inter-cluster communication

Central and site clusters now communicate via ClusterClient/
ClusterClientReceptionist instead of direct ActorSelection. Both
CentralCommunicationActor and SiteCommunicationActor are registered
with their cluster's receptionist. Central creates one ClusterClient
per site using NodeA/NodeB contact points from the DB. Sites configure
multiple CentralContactPoints for automatic failover between central
nodes. ISiteClientFactory enables test injection.

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -1,3 +1,4 @@
+using System.Collections.Immutable;
 using Akka.Actor;
 using Akka.Cluster;
 using Akka.Cluster.Tools.Client;
@@ -168,10 +169,15 @@ akka {{
     /// </summary>
     private void RegisterCentralActors()
     {
+        var siteClientFactory = new DefaultSiteClientFactory();
         var centralCommActor = _actorSystem!.ActorOf(
-            Props.Create(() => new CentralCommunicationActor(_serviceProvider)),
+            Props.Create(() => new CentralCommunicationActor(_serviceProvider, siteClientFactory)),
             "central-communication");
 
+        // Register CentralCommunicationActor with ClusterClientReceptionist so site ClusterClients can reach it
+        ClusterClientReceptionist.Get(_actorSystem).RegisterService(centralCommActor);
+        _logger.LogInformation("CentralCommunicationActor registered with ClusterClientReceptionist");
+
         // Wire up the CommunicationService with the actor reference
         var commService = _serviceProvider.GetService<CommunicationService>();
         commService?.SetCommunicationActor(centralCommActor);
@@ -247,26 +253,37 @@ akka {{
         var dmProxy = _actorSystem.ActorOf(proxyProps, "deployment-manager-proxy");
 
         // WP-4: Create SiteCommunicationActor for receiving messages from central
-        _actorSystem.ActorOf(
+        var siteCommActor = _actorSystem.ActorOf(
             Props.Create(() => new SiteCommunicationActor(
                 _nodeOptions.SiteId!,
                 _communicationOptions,
                 dmProxy)),
             "site-communication");
 
+        // Register SiteCommunicationActor with ClusterClientReceptionist so central ClusterClients can reach it
+        ClusterClientReceptionist.Get(_actorSystem).RegisterService(siteCommActor);
+
         _logger.LogInformation(
             "Site actors registered. DeploymentManager singleton scoped to role={SiteRole}, SiteCommunicationActor created.",
             siteRole);
 
-        // Register with Central if configured — tells Central where to send deployment commands
-        if (!string.IsNullOrWhiteSpace(_communicationOptions.CentralActorPath))
+        // Create ClusterClient to central if contact points are configured
+        if (_communicationOptions.CentralContactPoints.Count > 0)
         {
-            var siteCommActor = _actorSystem.ActorSelection("/user/site-communication");
-            siteCommActor.Tell(new RegisterCentralPath(_communicationOptions.CentralActorPath));
+            var contacts = _communicationOptions.CentralContactPoints
+                .Select(cp => ActorPath.Parse($"{cp}/system/receptionist"))
+                .ToImmutableHashSet();
+            var clientSettings = ClusterClientSettings.Create(_actorSystem)
+                .WithInitialContacts(contacts);
+            var centralClient = _actorSystem.ActorOf(
+                ClusterClient.Props(clientSettings), "central-cluster-client");
+
+            var siteCommSelection = _actorSystem.ActorSelection("/user/site-communication");
+            siteCommSelection.Tell(new RegisterCentralClient(centralClient));
 
             _logger.LogInformation(
-                "Configured central heartbeat path at {CentralPath} for site {SiteId}",
-                _communicationOptions.CentralActorPath, _nodeOptions.SiteId);
+                "Created ClusterClient to central with {Count} contact point(s) for site {SiteId}",
+                contacts.Count, _nodeOptions.SiteId);
         }
     }
 }

src/ScadaLink.Host/appsettings.Site.json

@@ -30,7 +30,10 @@
       "ReplicationEnabled": true
     },
     "Communication": {
-      "CentralActorPath": "akka.tcp://scadalink@localhost:8081/user/central-communication",
+      "CentralContactPoints": [
+        "akka.tcp://scadalink@localhost:8081",
+        "akka.tcp://scadalink@localhost:8082"
+      ],
       "DeploymentTimeout": "00:02:00",
       "LifecycleTimeout": "00:00:30",
       "QueryTimeout": "00:00:30",