Check an uploaded app capture: ready or rejected.

Streams the stored file once to hash it, reads it with ffprobe, and marks the capture ready (size, sha256, picture size, frame rate, length, sound) or rejected with rejectionReason: every way it falls short and the ffmpeg command that re-encodes it (a rejected capture's files are deleted at once). A capture is an MP4 screen recording of a web app: H.264, constant 30 fps, at most 1920×1080, tagged bt709 (its sound, when it has any, in AAC), up to 512 MB. Every file is checked after its upload; one that does not conform is rejected with the command that re-encodes it. A capture already checked is answered as it is, so no Idempotency-Key. Free.