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/HealthReportSender.cs

@@ -49,9 +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);
+                // Only the active node (running the DeploymentManager singleton) sends health reports.
+                // The standby node has no instance/connection data and would overwrite the active's report.
+                if (!_collector.IsActiveNode)
+                    continue;
 
                 var seq = Interlocked.Increment(ref _sequenceNumber);
                 var report = _collector.CollectReport(_siteId);