fix: only active singleton node sends health reports

Both nodes of a site cluster were sending health reports. The standby
node (without the DeploymentManager singleton) reported 0 instances and
no connections, overwriting the active node's data in the aggregator.

Added IsActiveNode flag to ISiteHealthCollector, set by
DeploymentManagerActor on PreStart/PostStop. HealthReportSender skips
sending when the node is not active. Also ensured EnsureDclConnections
is called during startup batch creation so data connections survive
container restarts.

src/ScadaLink.HealthMonitoring/SiteHealthCollector.cs

@@ -17,6 +17,7 @@ public class SiteHealthCollector : ISiteHealthCollector
     private readonly ConcurrentDictionary<string, TagResolutionStatus> _tagResolutionCounts = new();
     private IReadOnlyDictionary<string, int> _sfBufferDepths = new Dictionary<string, int>();
     private int _deployedInstanceCount, _enabledInstanceCount, _disabledInstanceCount;
+    private volatile bool _isActiveNode;
 
     /// <summary>
     /// Increment the script error counter. Covers unhandled exceptions,
@@ -90,6 +91,10 @@ public class SiteHealthCollector : ISiteHealthCollector
         Interlocked.Exchange(ref _disabledInstanceCount, disabled);
     }
 
+    public void SetActiveNode(bool isActive) => _isActiveNode = isActive;
+
+    public bool IsActiveNode => _isActiveNode;
+
     /// <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).