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/SiteHealthCollector.cs

@@ -15,6 +15,8 @@ public class SiteHealthCollector : ISiteHealthCollector
     private int _deadLetterCount;
     private readonly ConcurrentDictionary<string, ConnectionHealth> _connectionStatuses = new();
     private readonly ConcurrentDictionary<string, TagResolutionStatus> _tagResolutionCounts = new();
+    private IReadOnlyDictionary<string, int> _sfBufferDepths = new Dictionary<string, int>();
+    private int _deployedInstanceCount, _enabledInstanceCount, _disabledInstanceCount;
 
     /// <summary>
     /// Increment the script error counter. Covers unhandled exceptions,
@@ -68,6 +70,26 @@ public class SiteHealthCollector : ISiteHealthCollector
         _tagResolutionCounts[connectionName] = new TagResolutionStatus(totalSubscribed, successfullyResolved);
     }
 
+    /// <summary>
+    /// Set the current store-and-forward buffer depths snapshot.
+    /// Called before report collection with data from the S&amp;F service.
+    /// </summary>
+    public void SetStoreAndForwardDepths(IReadOnlyDictionary<string, int> depths)
+    {
+        _sfBufferDepths = depths;
+    }
+
+    /// <summary>
+    /// Set the current instance counts.
+    /// Called by the Deployment Manager after instance state changes.
+    /// </summary>
+    public void SetInstanceCounts(int deployed, int enabled, int disabled)
+    {
+        Interlocked.Exchange(ref _deployedInstanceCount, deployed);
+        Interlocked.Exchange(ref _enabledInstanceCount, enabled);
+        Interlocked.Exchange(ref _disabledInstanceCount, disabled);
+    }
+
     /// <summary>
     /// Collect the current health report for the site and reset interval counters.
     /// Connection statuses and tag resolution counts are NOT reset (they reflect current state).
@@ -84,8 +106,8 @@ public class SiteHealthCollector : ISiteHealthCollector
         var connectionStatuses = new Dictionary<string, ConnectionHealth>(_connectionStatuses);
         var tagResolution = new Dictionary<string, TagResolutionStatus>(_tagResolutionCounts);
 
-        // S&F buffer depth: placeholder (Phase 3C)
-        var sfBufferDepths = new Dictionary<string, int>();
+        // Snapshot current S&F buffer depths
+        var sfBufferDepths = new Dictionary<string, int>(_sfBufferDepths);
 
         return new SiteHealthReport(
             SiteId: siteId,
@@ -96,6 +118,9 @@ public class SiteHealthCollector : ISiteHealthCollector
             ScriptErrorCount: scriptErrors,
             AlarmEvaluationErrorCount: alarmErrors,
             StoreAndForwardBufferDepths: sfBufferDepths,
-            DeadLetterCount: deadLetters);
+            DeadLetterCount: deadLetters,
+            DeployedInstanceCount: _deployedInstanceCount,
+            EnabledInstanceCount: _enabledInstanceCount,
+            DisabledInstanceCount: _disabledInstanceCount);
     }
 }