feat: Add client.subscribe.servers hook for plugin-based server extension

- Add HookManager::filter('client.subscribe.servers') hook in ClientController::subscribe()
- Allow plugins to inject custom servers into subscription responses
- Update protocol classes to support extended server configurations
- Enable dynamic server list modification before protocol processing

app/Http/Controllers/V1/Client/ClientController.php

@@ -56,6 +56,7 @@ class ClientController extends Controller
             ?? General::class;
 
         $servers = ServerService::getAvailableServers($user);
+        $servers = HookManager::filter('client.subscribe.servers', $servers, $user, $request);
 
         $serversFiltered = $this->filterServers(
             servers: $servers,