Add settings endpoints and registration status check: implement new API endpoints for settings management, including registration status retrieval, and update the registration command handler to enforce registration rules based on site settings.

backend/src/TeleWave.Api/Program.cs

@@ -117,6 +117,7 @@ app.MapShowEndpoints();
 app.MapChannelEndpoints();
 app.MapStreamingEndpoints();
 app.MapMaintenanceEndpoints();
+app.MapSettingsEndpoints();
 
 // Раздача статики SPA из wwwroot + fallback на index.html для клиентских маршрутов.
 app.UseDefaultFiles();