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.StoreAndForward/ScadaLink.StoreAndForward.csproj

@@ -8,6 +8,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Akka" Version="1.5.62" />
     <PackageReference Include="Microsoft.Data.Sqlite" Version="10.0.7" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" Version="10.0.7" />
     <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="10.0.7" />