refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

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

Musify.Api/Extensions/ServiceCollectionExtensions.cs

@@ -0,0 +1,34 @@
+using FluentValidation;
+using Musify.Infrastructure.MassTransit;
+using Musify.Infrastructure.Persistence;
+using Musify.Application.Configuration;
+using Musify.Infrastructure.Services;
+using Musify.Api.Authentication;
+using Musify.Api.DataTransferObjects.PlayLists;
+using Musify.Api.OpenApi;
+using Musify.Api.Scalar;
+
+namespace Musify.Api.Extensions;
+
+public static class ServiceCollectionExtensions
+{
+    public static IServiceCollection AddApplicationServices(this IServiceCollection services, IConfiguration 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();
+        services.AddValidatorsFromAssemblyContaining<CreatePlayListRequest>();
+
+        return services;
+    }
+}