The watch frame alone read as "nothing related" — so a related column YouTube moved, or a list
drawn by renderers this does not read, answered an empty list instead of an error. Now only the
"isn't available" promo YouTube draws for a video it no longer has is an empty page; anything else
it cannot read is a ScrapeError.
The flattening of a video renderer and the read of the continuation token are one each, in
innertube, for the search and the related scrape.
GET /api/search/related/{video_id} reads the list YouTube shows beside a
video from the website's InnerTube "next" endpoint (no quota), and brings
it into the catalog the way live-search results come in. It leaves out
what the user watched or hid, here or in their Takeout history (unless
taken back to "new" since), blocked channels and the video itself, and
pages on until 20 are gathered (at most 4 pages). region=topical|hu picks
the locale the list is filled in. Its own per-user daily cap
(related_daily_limit_per_user, one event per opened list); a failed
scrape answers 422 youtube/unavailable.
The InnerTube request (live key, consent cookie, errors) moves into
youtube/innertube.py, shared by the search scrape and the related one.
peter