Save answers to one questionnaire answer request

Writes a batch of answers against a single answer request — the write behind every answer a supplier saves, the "not available" toggle, the bulk auto-fill from the company profile and the SLCP prefill. Only the company the request was addressed to may write to it; anyone else gets 404 request_not_found, so the existence of another company's request is never confirmed. A missing question answers 404 question_not_found, and an answer that fails its Retraced datapoint answers 400 invalid_answer_for_datapoint. On a CERTIFICATE question each linked certificate must be the one its option asks for — a certificate certifying a different standard answers 400 certificate_does_not_match_option, naming the certificate and both standards. The check only fires when the option and the certificate are both known and disagree, so an option this question does not have, an option carrying no standard, or a missing certificate all pass. Every answer is validated before any is written, so a batch containing one bad answer stores nothing; past that point the batch is not transactional, and an answer that fails after earlier ones have committed leaves those in place. Re-answering the same question updates the stored answer rather than adding a second one, keeping its id and any reviewer verdict, so re-submitting is a silent no-op by design. Answers omitting status default to IN_REVIEW. Answers to photo, video and audit questions also register their uploaded files and move them into private storage. Responds 201 with an empty body.