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/models.go

@@ -85,6 +85,10 @@ type Library struct {
 	AutotagWarningAcked int64
 }
 
+type LyricsIndex struct {
+	Lyrics string
+}
+
 type PlayHistory struct {
 	ID          int64
 	AudioFileID int64
@@ -100,12 +104,13 @@ type PlayerState struct {
 }
 
 type Playlist struct {
-	ID         int64
-	Name       string
-	IsSmart    int64
-	SmartRules sql.NullString
-	CreatedAt  time.Time
-	UpdatedAt  time.Time
+	ID              int64
+	Name            string
+	IsSmart         int64
+	SmartRules      sql.NullString
+	SmartSnapshotAt sql.NullTime
+	CreatedAt       time.Time
+	UpdatedAt       time.Time
 }
 
 type PlaylistTrack struct {
@@ -184,6 +189,12 @@ type SearchIndex struct {
 	Album    string
 }
 
+type TaggingCandidate struct {
+	GroupKey   string
+	Candidates string
+	ComputedAt time.Time
+}
+
 type TaggingItem struct {
 	GroupKey             string
 	LibraryID            int64