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.Host/Program.cs

@@ -130,6 +130,7 @@ try
             ResponseWriter = UIResponseWriter.WriteHealthCheckUIResponse
         });
 
+        app.MapStaticAssets();
         app.MapCentralUI<ScadaLink.Host.Components.App>();
         app.MapInboundAPI();
         await app.RunAsync();