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. 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.