- search_youtube, fetch_trending, fetch_featured_channels now use _meta_run
- Replaced ThreadPoolExecutor(4) parallel searches with sequential loop
- Replaced ThreadPoolExecutor(3) parallel featured-channel fetches with sequential
- _fetch_and_index_channel passes polite=True to fetch_channel/video_metadata
Discovery was firing 4+ simultaneous yt-dlp processes, each with cookies,
which is what invalidated the session.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>