fix: update instance counts after startup batch creation completes

UpdateInstanceCounts() was only called before Instance Actors were
created (in HandleStartupConfigsLoaded), showing 0 enabled on the
health dashboard. Now also called after each batch in
HandleStartNextBatch to reflect actual running actor count.

src/ScadaLink.SiteRuntime/Actors/DeploymentManagerActor.cs

@@ -163,6 +163,8 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
             CreateInstanceActor(config.InstanceUniqueName, config.ConfigJson);
         }
 
+        UpdateInstanceCounts();
+
         // Schedule next batch if there are more, using Timers (IWithTimers)
         if (endIdx < state.Configs.Count)
         {