Add a live YouTube search that materialises results into the shared catalog so
they render with the normal feed cards + in-app player and gain per-user state.
- YouTubeClient.search_videos(): search.list (100 units), embeddable-only, returns
flat stubs + nextPageToken; surfaces liveBroadcastContent for live filtering.
- routes/search.py GET /api/search/youtube: require_human + per-user daily cap
(search_daily_limit_per_user, default 70) + can_spend pre-check (429 on either);
drops live/upcoming, upserts channel stubs (channels.list) + video stubs, enriches
(videos.list), runs the youtube.com/shorts probe, then excludes Shorts/live and
returns feed cards in relevance order with the YouTube pageToken as the cursor.
- Provenance: videos.via_search / channels.from_search (migration 0028) flag
search-discovered rows; the feed hides them from the Library (scope=all) by default
via exclude_search_discovered, leaving the Mine feed untouched.
- quota.actions_today() counts a user's per-action events today for the cap; only the
search.list call is attributed VIDEOS_SEARCH so the counter is exactly 1 per search.
9b1bdb6b42