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/events/events.go

@@ -90,4 +90,22 @@ const (
 // Explore / search index events.
 const (
 	IndexStatusChanged = "IndexStatusChanged"
+
+	// ArtistDiscographyReady fires (payload: artist MBID string) after a
+	// lazy background discography fetch persists into the index, so the
+	// artist detail page can re-fetch its top tracks / top releases
+	// without the initial request having blocked on a live fetch.
+	ArtistDiscographyReady = "ArtistDiscographyReady"
+
+	// ArtistSimilarReady fires (payload: artist MBID string) after a lazy
+	// background similar-artists fetch persists into similar_artist_map,
+	// so the artist detail page can re-fetch that section without the
+	// initial request having blocked on a live LB labs call.
+	ArtistSimilarReady = "ArtistSimilarReady"
+
+	// AlbumReleasesReady fires (payload: release-group MBID string) after a
+	// lazy background BrowseReleases fetch populates the response cache, so
+	// the album detail page can re-fetch its versions / tracklist without
+	// the initial request having blocked on a live MusicBrainz browse.
+	AlbumReleasesReady = "AlbumReleasesReady"
 )