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/Program.cs

@@ -11,6 +11,8 @@ builder.Services
     .AddApplicationServices(builder.Configuration)
     .AddObservability(builder.Configuration);
 
+builder.Services.AddProblemDetails();
+
 if (builder.Environment.IsDevelopment())
 {
     builder.Services.AddCors(options => options.AddPolicy(devCorsPolicy, policy =>
@@ -19,6 +21,8 @@ if (builder.Environment.IsDevelopment())
 
 var app = builder.Build();
 
+app.UseExceptionHandler();
+
 if (app.Environment.IsDevelopment())
 {
     app.UseCors(devCorsPolicy);