refactor(di): add AddApplication entry point and generic AddValidatedOptions

- Add Musify.Application.AddApplication(services, config) as the layer's DI
  entry point: registers Mediator, UploadIntentValidator and all app configs
- Collapse the 5 near-identical AddXConfiguration methods into a single
  generic AddValidatedOptions<T>(config, section)
- Move UploadIntentValidator registration out of Infrastructure into the
  Application layer (fixes layering leak); Infra config binding now uses the
  generic helper
- Api calls AddApplication; Worker uses AddValidatedOptions for its configs

Build: 0 warnings, 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.Api/Extensions/ServiceCollectionExtensions.cs

@@ -1,7 +1,7 @@
 using FluentValidation;
 using Musify.Infrastructure.MassTransit;
 using Musify.Infrastructure.Persistence;
-using Musify.Application.Configuration;
+using Musify.Application;
 using Musify.Infrastructure.Services;
 using Musify.Api.Authentication;
 using Musify.Api.DataTransferObjects.PlayLists;
@@ -14,16 +14,11 @@ public static class ServiceCollectionExtensions
 {
     public static IServiceCollection AddApplicationServices(this IServiceCollection services, IConfiguration configuration)
     {
+        services.AddApplication(configuration);
         services.AddDatabase(configuration);
-        services.AddApplicationStorageConfiguration(configuration);
-        services.AddPlayListConfiguration(configuration);
-        services.AddTrackConfiguration(configuration);
-        services.AddStreamGatewayConfiguration(configuration);
         services.AddStorageService(configuration);
         services.AddStreamTicketService(configuration);
-        services.AddUploadIntentConfiguration(configuration);
         services.AddMassTransitClient(configuration);
-        services.AddMediator();
         services.AddAuthenticationConfiguration(configuration);
         services.AddOpenApiConfiguration();
         services.AddScalarConfiguration();