Create a demo-video upload URL

For the talking-head (Video Hook & Demo) format. PUT the video bytes to uploadUrl with the returned contentType as the Content-Type header (it is signed), then pass key to POST /content/jobs as demoVideoKey (or hookVideoKey). The file is checked when the job is created: mp4, mov or webm, at most 100 MB, a demo at most 60 s and a hook at most 15 s; a rejected upload is deleted. The render replaces the recording's own audio with a library music track (assetFilters.audioMood; none = silent).