Start Media Upload

Open a RESUMABLE upload for a coursework video or audio file.

{content_type, size, filename?, org_id?} -> {mode: "resumable", session_url, object_path, chunk_bytes, ...}. The browser sends the file to session_url in chunk_bytes pieces, asks the session how much landed whenever a piece fails, and carries on from there; then it calls /upload/video/complete with object_path, exactly as the single-PUT rail does. Nothing passes through the pod.

Same gates as /upload/video/sign: a media type we take, a size within the cap, a caller who belongs to a school (to the named one, if named). The session is minted for an object under THIS caller's prefix, so /complete refuses to record it for anybody else.