fix(store-and-forward): wire up parked-message handler and start S&F service on sites

The Parked Messages page returned "Parked message handler not available"
because no actor was ever registered for ParkedMessages, and Retry/Discard
requests had no Receive at all (would have hit deadletters). On top of
that, StoreAndForwardService.StartAsync() was never called anywhere, so
the sf_messages SQLite table was never created and the retry timer never
ran — silently breaking all of S&F.

- New ParkedMessageHandlerActor bridges StoreAndForwardService.{Get,Retry,Discard}
  using the Sender→Task→PipeTo pattern already used in DeploymentManagerActor.
- SiteCommunicationActor now routes ParkedMessageRetryRequest and
  ParkedMessageDiscardRequest the same way as the existing Query handler.
- AkkaHostedService.RegisterSiteActors() resolves StoreAndForwardService,
  calls StartAsync() to create the schema and start the timer, then
  creates and registers the handler actor.

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -316,6 +316,21 @@ akka {{
             siteCommActor.Tell(new RegisterLocalHandler(LocalHandlerType.EventLog, eventLogHandler));
         }
 
+        // Parked message handler — bridges Akka to StoreAndForwardService
+        var storeAndForwardService = _serviceProvider.GetService<StoreAndForwardService>();
+        if (storeAndForwardService != null)
+        {
+            // Initialize SQLite schema and start the retry timer. Must complete before
+            // any actor or HTTP handler touches the service.
+            storeAndForwardService.StartAsync().GetAwaiter().GetResult();
+
+            var parkedMessageHandler = _actorSystem.ActorOf(
+                Props.Create(() => new ParkedMessageHandlerActor(
+                    storeAndForwardService, _nodeOptions.SiteId!)),
+                "parked-message-handler");
+            siteCommActor.Tell(new RegisterLocalHandler(LocalHandlerType.ParkedMessages, parkedMessageHandler));
+        }
+
         // Register SiteCommunicationActor with ClusterClientReceptionist so central ClusterClients can reach it
         ClusterClientReceptionist.Get(_actorSystem).RegisterService(siteCommActor);