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.

tests/ScadaLink.HealthMonitoring.Tests/CentralHealthAggregatorTests.cs

@@ -50,7 +50,10 @@ public class CentralHealthAggregatorTests
             ScriptErrorCount: 0,
             AlarmEvaluationErrorCount: 0,
             StoreAndForwardBufferDepths: new Dictionary<string, int>(),
-            DeadLetterCount: 0);
+            DeadLetterCount: 0,
+            DeployedInstanceCount: 0,
+            EnabledInstanceCount: 0,
+            DisabledInstanceCount: 0);
 
     [Fact]
     public void ProcessReport_StoresState_ForNewSite()