Live YouTube search can now use YouTube's internal InnerTube endpoint instead
of search.list, materialising results through the same enrich/provenance path
at zero API quota (search.list costs 100 units/page; scrape costs nothing, only
the cheap shared videos.list enrich is charged).
- youtube/search_scrape.py: InnerTube search returning the same page shape as
YouTubeClient.search_videos (items + continuation cursor); SOCS consent cookie,
videoRenderer walk (channels/playlists/Shorts shelves naturally excluded),
type:video filter, cached InnerTube key/version with constant fallbacks.
- routes/search.py: admin-selectable source (search_source, default scrape).
Scrape path skips the 100-unit budget pre-check and logs a zero-cost search
event so the per-user daily cap still counts it; api path unchanged. Response
carries the active source.
- sysconfig/config: new search_source key (scrape|api).
- quota.log_action: record a zero-cost action event for per-user rate limits.
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