Inconsistent/absent error response contract (not RFC 7807)

[umerfaruk]

Summary

Error responses have no consistent shape across the API.

Details

Depending on which middleware/handler catches the exception, clients get: a JSON `{ "message": ... }` body, plain text, or an empty body with just a status code (see #10).

Suggested fix

Standardize on `application/problem+json` (`ProblemDetails`) so clients get a predictable, typed error shape (`type`/`title`/`status`/`detail`). Best done together with #10.