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.ConfigurationDatabase/Configurations/SiteConfiguration.cs

@@ -21,6 +21,9 @@ public class SiteConfiguration : IEntityTypeConfiguration<Site>
         builder.Property(s => s.Description)
             .HasMaxLength(2000);
 
+        builder.Property(s => s.NodeAAddress).HasMaxLength(500);
+        builder.Property(s => s.NodeBAddress).HasMaxLength(500);
+
         builder.HasIndex(s => s.Name).IsUnique();
         builder.HasIndex(s => s.SiteIdentifier).IsUnique();
     }