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.Commons/Entities/Sites/Site.cs

@@ -6,6 +6,8 @@ public class Site
     public string Name { get; set; }
     public string SiteIdentifier { get; set; }
     public string? Description { get; set; }
+    public string? NodeAAddress { get; set; }
+    public string? NodeBAddress { get; set; }
 
     public Site(string name, string siteIdentifier)
     {