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.Communication/CommunicationOptions.cs

@@ -28,10 +28,10 @@ public class CommunicationOptions
     public TimeSpan HealthReportTimeout { get; set; } = TimeSpan.FromSeconds(10);
 
     /// <summary>
-    /// Remote actor path for the central communication actor. Used by site nodes to
-    /// register with central on startup (e.g. "akka.tcp://scadalink@central:8081/user/central-communication").
+    /// Contact point addresses for the central cluster (e.g. "akka.tcp://scadalink@central-a:8081").
+    /// Used by site nodes to create a ClusterClient for reaching central.
     /// </summary>
-    public string? CentralActorPath { get; set; }
+    public List<string> CentralContactPoints { get; set; } = new();
 
     /// <summary>Akka.Remote transport heartbeat interval.</summary>
     public TimeSpan TransportHeartbeatInterval { get; set; } = TimeSpan.FromSeconds(5);