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.SiteRuntime/Actors/ScriptExecutionActor.cs

@@ -2,6 +2,7 @@ using Akka.Actor;
 using Microsoft.CodeAnalysis.Scripting;
 using Microsoft.Extensions.Logging;
 using ScadaLink.Commons.Messages.ScriptExecution;
+using ScadaLink.HealthMonitoring;
 using ScadaLink.SiteRuntime.Scripts;
 
 namespace ScadaLink.SiteRuntime.Actors;
@@ -28,7 +29,8 @@ public class ScriptExecutionActor : ReceiveActor
         SiteRuntimeOptions options,
         IActorRef replyTo,
         string correlationId,
-        ILogger logger)
+        ILogger logger,
+        ISiteHealthCollector? healthCollector = null)
     {
         // Immediately begin execution
         var self = Self;
@@ -37,7 +39,7 @@ public class ScriptExecutionActor : ReceiveActor
         ExecuteScript(
             scriptName, instanceName, compiledScript, parameters, callDepth,
             instanceActor, sharedScriptLibrary, options, replyTo, correlationId,
-            self, parent, logger);
+            self, parent, logger, healthCollector);
     }
 
     private static void ExecuteScript(
@@ -53,7 +55,8 @@ public class ScriptExecutionActor : ReceiveActor
         string correlationId,
         IActorRef self,
         IActorRef parent,
-        ILogger logger)
+        ILogger logger,
+        ISiteHealthCollector? healthCollector)
     {
         var timeout = TimeSpan.FromSeconds(options.ScriptExecutionTimeoutSeconds);
 
@@ -93,6 +96,7 @@ public class ScriptExecutionActor : ReceiveActor
             }
             catch (OperationCanceledException)
             {
+                healthCollector?.IncrementScriptError();
                 var errorMsg = $"Script '{scriptName}' on instance '{instanceName}' timed out after {timeout.TotalSeconds}s";
                 logger.LogWarning(errorMsg);
 
@@ -105,6 +109,7 @@ public class ScriptExecutionActor : ReceiveActor
             }
             catch (Exception ex)
             {
+                healthCollector?.IncrementScriptError();
                 // WP-32: Failures logged to site event log; script NOT disabled after failure
                 var errorMsg = $"Script '{scriptName}' on instance '{instanceName}' failed: {ex.Message}";
                 logger.LogError(ex, "Script execution failed: {Script} on {Instance}", scriptName, instanceName);