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.Communication/Actors/SiteCommunicationActor.cs

@@ -136,6 +136,28 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
             }
         });
 
+        Receive<ParkedMessageRetryRequest>(msg =>
+        {
+            if (_parkedMessageHandler != null)
+                _parkedMessageHandler.Forward(msg);
+            else
+            {
+                Sender.Tell(new ParkedMessageRetryResponse(
+                    msg.CorrelationId, false, "Parked message handler not available"));
+            }
+        });
+
+        Receive<ParkedMessageDiscardRequest>(msg =>
+        {
+            if (_parkedMessageHandler != null)
+                _parkedMessageHandler.Forward(msg);
+            else
+            {
+                Sender.Tell(new ParkedMessageDiscardResponse(
+                    msg.CorrelationId, false, "Parked message handler not available"));
+            }
+        });
+
         // Internal: send heartbeat tick
         Receive<SendHeartbeat>(_ => SendHeartbeatToCentral());