[T-057] /v1/ocr silently ignores Mistral-OCR request parameters it doesn't support

`pages`, `include_image_base64`, `image_limit`, `table_format`, annotation formats etc. are accepted and ignored — clients get a 200 with different semantics than requested. Fail loud (422 naming the parameter) until each is implemented.

Canonical ticket: [T-057](https://github.com/Ztein/figmark/blob/main/docs/tickets/T-057-ocr-endpoint-silently-ignores-request-parameters.md)