feat: wire all health metrics and add instance counts to dashboard

Wired ISiteHealthCollector calls for script errors (ScriptExecutionActor),
alarm eval errors (AlarmActor), dead letters (DeadLetterMonitorActor), and
S&F buffer depth placeholder. Added instance count tracking (deployed/
enabled/disabled) to SiteHealthReport via DeploymentManagerActor. Updated
Health Dashboard UI to show instance counts per site. All metrics flow
through the existing health report pipeline via ClusterClient.

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -118,8 +118,9 @@ akka {{
         // Register the dead letter monitor actor
         var loggerFactory = _serviceProvider.GetRequiredService<ILoggerFactory>();
         var dlmLogger = loggerFactory.CreateLogger<DeadLetterMonitorActor>();
+        var dlmHealthCollector = _serviceProvider.GetService<ScadaLink.HealthMonitoring.ISiteHealthCollector>();
         _actorSystem.ActorOf(
-            Props.Create(() => new DeadLetterMonitorActor(dlmLogger)),
+            Props.Create(() => new DeadLetterMonitorActor(dlmLogger, dlmHealthCollector)),
             "dead-letter-monitor");
 
         // Register role-specific actors
@@ -227,6 +228,9 @@ akka {{
             _logger.LogInformation("Data Connection Layer manager actor created");
         }
 
+        // Resolve the health collector for the Deployment Manager
+        var siteHealthCollector = _serviceProvider.GetService<ScadaLink.HealthMonitoring.ISiteHealthCollector>();
+
         // Create the Deployment Manager as a cluster singleton
         var singletonProps = ClusterSingletonManager.Props(
             singletonProps: Props.Create(() => new DeploymentManagerActor(
@@ -236,7 +240,8 @@ akka {{
                 streamManager,
                 siteRuntimeOptionsValue,
                 dmLogger,
-                dclManager)),
+                dclManager,
+                siteHealthCollector)),
             terminationMessage: PoisonPill.Instance,
             settings: ClusterSingletonManagerSettings.Create(_actorSystem!)
                 .WithRole(siteRole)