Upload the video of a video scan through the API

Only when upload.url points here (no direct upload link available; always for a test scan, whose video is counted and dropped, never stored). Content-Length and Content-Type must match what was declared, and the body must be that size. At most 99614720 bytes (95 MiB): a bigger video answers 413 FILE_TOO_LARGE and goes to the direct upload link instead. Needs scans:write.