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/Extensions/ErrorOrHttpExtensions.cs

@@ -15,6 +15,11 @@ public static class ErrorOrHttpExtensions
             : Results.Ok(result.Value);
     }
 
+    public static IResult ToCreatedResult<T>(this ErrorOr<T> result, Func<T, string> locationFactory) =>
+        result.IsError
+            ? Problem(result.Errors)
+            : Results.Created(locationFactory(result.Value), result.Value);
+
     private static IResult Problem(List<Error> errors)
     {
         if (errors.All(e => e.Type == ErrorType.Validation))