refactor(api): OpenAPI response metadata, cached user sync, ProblemDetails

- Document every endpoint's responses (Produces<T>/status codes/validation)
  so the generated OpenAPI/Scalar docs carry response schemas and status codes
  instead of empty responses
- Cache recently-synced user ids (IMemoryCache, 15 min) in the JWT
  TokenValidated handler so user provisioning no longer hits the DB on every
  authenticated request
- Add ProblemDetails + UseExceptionHandler for consistent RFC7807 errors
- Add ToCreatedResult ErrorOr helper and use it in the create endpoints
- Normalize the playlists route to lowercase (/playlists)

Verified: booted the Api against RabbitMQ + Postgres in Docker and inspected
/openapi/v1.json — operations now expose response schemas/status codes and the
route is /playlists.

Build: 0 warnings, 0 errors.

backend/Musify.Api/Authentication/AuthenticationExtensions.cs

@@ -17,6 +17,7 @@ public static class AuthenticationExtensions
         services.AddSingleton(serviceProvider =>
             serviceProvider.GetRequiredService<IOptions<AuthenticationConfiguration>>().Value);
 
+        services.AddMemoryCache();
         services.AddScoped<JwtBearerEventsHandler>();
         services.ConfigureOptions<JwtBearerOptionsSetup>();