feat: autotag scoring overhaul, dump-based explore index, and lyrics search

Consolidates in-progress work across autotag, explore, and library:

- autotag: beets/Picard-informed scoring engine — ID-first matching, VA
  handling, recommendation tiers, and a merged distance/rank cascade, with
  an eval harness for regression tracking.
- explore: offline MusicBrainz dump import/incremental refresh replaces the
  legacy tier crawl; index-first local search with fuzzy matching and a
  dedicated ranker; disk-free guards for dump downloads.
- library: artist-credit extraction and matching.
- lyrics: owned-library lyric search (FTS) with LRCLIB backfill.

Also: rewrite README to be user-focused, and migrate upstream to
git.ljones.me/yonlu/yellowjacket.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/database/sql/sqlcgen/playlists.sql.go

@@ -82,7 +82,7 @@ func (q *Queries) CountPlaylistsByName(ctx context.Context, name string) (int64,
 
 const createPlaylist = `-- name: CreatePlaylist :one
 INSERT INTO playlists (name) VALUES (?)
-RETURNING id, name, is_smart, smart_rules, created_at, updated_at
+RETURNING id, name, is_smart, smart_rules, smart_snapshot_at, created_at, updated_at
 `
 
 func (q *Queries) CreatePlaylist(ctx context.Context, name string) (Playlist, error) {
@@ -93,6 +93,7 @@ func (q *Queries) CreatePlaylist(ctx context.Context, name string) (Playlist, er
 		&i.Name,
 		&i.IsSmart,
 		&i.SmartRules,
+		&i.SmartSnapshotAt,
 		&i.CreatedAt,
 		&i.UpdatedAt,
 	)
@@ -205,7 +206,7 @@ func (q *Queries) GetAllPlaylistTracksWithMetadata(ctx context.Context) ([]GetAl
 }
 
 const getAllPlaylists = `-- name: GetAllPlaylists :many
-SELECT id, name, is_smart, smart_rules, created_at, updated_at FROM playlists ORDER BY updated_at DESC
+SELECT id, name, is_smart, smart_rules, smart_snapshot_at, created_at, updated_at FROM playlists ORDER BY updated_at DESC
 `
 
 func (q *Queries) GetAllPlaylists(ctx context.Context) ([]Playlist, error) {
@@ -222,6 +223,7 @@ func (q *Queries) GetAllPlaylists(ctx context.Context) ([]Playlist, error) {
 			&i.Name,
 			&i.IsSmart,
 			&i.SmartRules,
+			&i.SmartSnapshotAt,
 			&i.CreatedAt,
 			&i.UpdatedAt,
 		); err != nil {
@@ -251,7 +253,7 @@ func (q *Queries) GetNextPlaylistTrackPosition(ctx context.Context, playlistID i
 }
 
 const getPlaylist = `-- name: GetPlaylist :one
-SELECT id, name, is_smart, smart_rules, created_at, updated_at FROM playlists WHERE id = ? LIMIT 1
+SELECT id, name, is_smart, smart_rules, smart_snapshot_at, created_at, updated_at FROM playlists WHERE id = ? LIMIT 1
 `
 
 func (q *Queries) GetPlaylist(ctx context.Context, id int64) (Playlist, error) {
@@ -262,6 +264,7 @@ func (q *Queries) GetPlaylist(ctx context.Context, id int64) (Playlist, error) {
 		&i.Name,
 		&i.IsSmart,
 		&i.SmartRules,
+		&i.SmartSnapshotAt,
 		&i.CreatedAt,
 		&i.UpdatedAt,
 	)