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)

src/ScadaLink.Host/Actors/DeadLetterMonitorActor.cs

@@ -1,6 +1,7 @@
 using Akka.Actor;
 using Akka.Event;
 using Microsoft.Extensions.Logging;
+using ScadaLink.HealthMonitoring;
 
 namespace ScadaLink.Host.Actors;
 
@@ -11,12 +12,16 @@ namespace ScadaLink.Host.Actors;
 public class DeadLetterMonitorActor : ReceiveActor
 {
     private long _deadLetterCount;
+    private readonly ISiteHealthCollector? _healthCollector;
 
-    public DeadLetterMonitorActor(ILogger<DeadLetterMonitorActor> logger)
+    public DeadLetterMonitorActor(ILogger<DeadLetterMonitorActor> logger, ISiteHealthCollector? healthCollector = null)
     {
+        _healthCollector = healthCollector;
+
         Receive<DeadLetter>(dl =>
         {
             _deadLetterCount++;
+            _healthCollector?.IncrementDeadLetter();
             logger.LogWarning(
                 "Dead letter: {MessageType} from {Sender} to {Recipient}",
                 dl.Message.GetType().Name,