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.Application/ApplicationDependencyInjection.cs

@@ -0,0 +1,24 @@
+using Mediator;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Musify.Application.Configuration;
+using Musify.Application.Services;
+
+namespace Musify.Application;
+
+public static class ApplicationDependencyInjection
+{
+    public static IServiceCollection AddApplication(this IServiceCollection services, IConfiguration configuration)
+    {
+        services.AddMediator();
+        services.AddScoped<UploadIntentValidator>();
+
+        services.AddValidatedOptions<ApplicationStorageConfiguration>(configuration, ApplicationStorageConfiguration.SectionName);
+        services.AddValidatedOptions<PlayListConfiguration>(configuration, PlayListConfiguration.SectionName);
+        services.AddValidatedOptions<TrackConfiguration>(configuration, TrackConfiguration.SectionName);
+        services.AddValidatedOptions<StreamGatewayConfiguration>(configuration, StreamGatewayConfiguration.SectionName);
+        services.AddValidatedOptions<UploadIntentConfiguration>(configuration, UploadIntentConfiguration.SectionName);
+
+        return services;
+    }
+}