youtubeVideoId(): string | null
Returns the id of the video a YouTube URL points at, in any of the shapes
people paste (youtu.be/ID, /watch?v=ID, /embed/ID, /shorts/ID,
/live/ID), and null for any other URL. host is lower case, without
"www.". The canonical key drops the timestamp, since the same video linked
at 2:30 is still a repost; the YouTube provider reads it separately.