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/OpenApi/OpenApiExtensions.cs

@@ -0,0 +1,14 @@
+using Microsoft.AspNetCore.OpenApi;
+
+namespace Musify.Api.OpenApi;
+
+public static class OpenApiExtensions
+{
+    public static IServiceCollection AddOpenApiConfiguration(this IServiceCollection services)
+    {
+        services.AddOpenApi();
+        services.ConfigureOptions<OpenApiOptionsSetup>();
+
+        return services;
+    }
+}