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.

backend/Musify.Infrastructure/Services/ServicesDependencyInjection.cs

@@ -5,7 +5,6 @@ using Microsoft.Extensions.Options;
 using Musify.Application.Configuration;
 using Musify.Infrastructure.Configuration;
 using Musify.Infrastructure.Jobs;
-using Musify.Application.Services;
 using Musify.Application.Contracts;
 
 namespace Musify.Infrastructure.Services
@@ -46,15 +45,7 @@ namespace Musify.Infrastructure.Services
 
         public static IServiceCollection AddUploadIntentConfiguration(this IServiceCollection serviceDescriptors, IConfiguration configuration)
         {
-            serviceDescriptors
-                .AddOptionsWithValidateOnStart<UploadIntentConfiguration>()
-                .Bind(configuration.GetRequiredSection(UploadIntentConfiguration.SectionName))
-                .ValidateDataAnnotations();
-
-            serviceDescriptors.AddSingleton(serviceProvider =>
-                serviceProvider.GetRequiredService<IOptions<UploadIntentConfiguration>>().Value);
-
-            serviceDescriptors.AddScoped<UploadIntentValidator>();
+            serviceDescriptors.AddValidatedOptions<UploadIntentConfiguration>(configuration, UploadIntentConfiguration.SectionName);
 
             return serviceDescriptors;
         }