Adds 'discover' as an accepted system_variant on the playlists
table — alongside 'for_you' and 'songs_like_artist' — and three
sqlc bucket queries that back the new playlist's daily candidate
selection.
The three buckets surface tracks the user hasn't played and
hasn't liked:
- Dormant artists: artists this user has played < 10 times total.
Surfaces the long tail of their library.
- Cross-user likes: tracks any OTHER user has liked but this one
hasn't engaged with. Empty on single-user servers; the Go-side
allocator redistributes the deficit across the other two
buckets.
- Random unheard: pure random sample as the safety net and the
cold-start fallback.
All three share exclusion filters: not-played by this user, not
liked by this user, not in lidarr_quarantine for this user. All
order by md5(track_id || dateStr) for daily determinism — same
pattern as the existing tieBreakHash logic in system.go.
LIMIT values are generous (80/60/200) so the per-album (<=2) /
per-artist (<=3) caps and slot redistribution in T2 have headroom.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
