POST /api/video-marks/
Returns an id and a 15-minute presigned PUT for users/{user_id}/video-marks/{id}/input.mp4. You upload the bytes. The object stays private.
Required headers:
Authorization: Bearer <YOUR_AKAPULU_API_KEY>Content-Type: application/json
PUT the file to upload_url with Content-Type: video/mp4. Then read the mark.
The file must be an mp4 of at most 200MB. A different content type on this call is 400 VALIDATION.
