feat: replace site registration with database-driven site addressing

Central now resolves site Akka remoting addresses from the Sites DB table
(NodeAAddress/NodeBAddress) instead of relying on runtime RegisterSite
messages. Eliminates the race condition where sites starting before central
had their registration dead-lettered. Addresses are cached in
CentralCommunicationActor with 60s periodic refresh and on-demand refresh
when sites are added/edited/deleted via UI or CLI.

src/ScadaLink.Communication/CommunicationService.cs

@@ -9,6 +9,7 @@ using ScadaLink.Commons.Messages.InboundApi;
 using ScadaLink.Commons.Messages.Integration;
 using ScadaLink.Commons.Messages.Lifecycle;
 using ScadaLink.Commons.Messages.RemoteQuery;
+using ScadaLink.Communication.Actors;
 
 namespace ScadaLink.Communication;
 
@@ -39,6 +40,14 @@ public class CommunicationService
         _centralCommunicationActor = centralCommunicationActor;
     }
 
+    /// <summary>
+    /// Triggers an immediate refresh of the site address cache from the database.
+    /// </summary>
+    public void RefreshSiteAddresses()
+    {
+        GetActor().Tell(new RefreshSiteAddresses());
+    }
+
     private IActorRef GetActor()
     {
         return _centralCommunicationActor