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/testhelper.go

@@ -75,10 +75,13 @@ func NewTestDB(t *testing.T) *DB {
 	t.Cleanup(func() { _ = db.Close() })
 
 	return &DB{
-		db:      db,
-		Ctx:     ctx,
-		Queries: queries,
-		logger:  slog.Default(),
+		db:  db,
+		Ctx: ctx,
+		// The in-memory test DB shares one connection, so reads and
+		// writes use the same handle; ReadQueries aliases Queries.
+		Queries:     queries,
+		ReadQueries: queries,
+		logger:      slog.Default(),
 	}
 }