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.HealthMonitoring/HealthReportSender.cs

@@ -49,6 +49,10 @@ public class HealthReportSender : BackgroundService
         {
             try
             {
+                // TODO: Wire S&F buffer depths when StoreAndForward service is available in DI
+                // e.g., var depths = await _bufferDepthProvider.GetDepthsAsync();
+                //        _collector.SetStoreAndForwardDepths(depths);
+
                 var seq = Interlocked.Increment(ref _sequenceNumber);
                 var report = _collector.CollectReport(_siteId);