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/explore/lrclib_test.go

@@ -0,0 +1,127 @@
+package explore
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"yellowjacket/backend/database"
+)
+
+// newTestLRCLib builds an LRCLIB client whose HTTP calls hit the given
+// test server, sharing a real (in-memory) cache so caching behaviour is
+// exercised.
+func newTestLRCLib(t *testing.T, handler http.HandlerFunc) (*LRCLibClient, *httptest.Server) {
+	t.Helper()
+
+	srv := httptest.NewServer(handler)
+	t.Cleanup(srv.Close)
+
+	db := database.NewTestDB(t)
+	cache := NewCache(db, testLogger())
+
+	c := NewLRCLibClient(cache, testLogger())
+	// Point the client at the test server instead of the real API by
+	// overriding its transport to rewrite the host.
+	c.http = srv.Client()
+	c.baseURL = srv.URL
+
+	return c, srv
+}
+
+func TestLRCLibGetLyrics(t *testing.T) {
+	t.Parallel()
+
+	var calls int
+
+	c, _ := newTestLRCLib(t, func(w http.ResponseWriter, r *http.Request) {
+		calls++
+
+		if r.URL.Path != "/api/get" {
+			http.Error(w, "not found", http.StatusNotFound)
+
+			return
+		}
+
+		if r.URL.Query().Get("track_name") == "Missing" {
+			http.Error(w, "not found", http.StatusNotFound)
+
+			return
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{
+			"id": 42,
+			"trackName": "Yesterday",
+			"artistName": "The Beatles",
+			"albumName": "Help!",
+			"duration": 125,
+			"instrumental": false,
+			"plainLyrics": "Yesterday, all my troubles seemed so far away",
+			"syncedLyrics": "[00:00.00] Yesterday"
+		}`))
+	})
+
+	t.Run("hit returns plain + synced lyrics", func(t *testing.T) {
+		lyrics, err := c.GetLyrics(context.Background(), "The Beatles", "Yesterday", "Help!", 125)
+		if err != nil {
+			t.Fatalf("GetLyrics: %v", err)
+		}
+
+		if lyrics.Plain == "" || lyrics.Synced == "" {
+			t.Errorf("expected plain and synced lyrics, got %+v", lyrics)
+		}
+
+		if lyrics.Instrumental {
+			t.Error("expected non-instrumental")
+		}
+	})
+
+	t.Run("second identical request is served from cache", func(t *testing.T) {
+		before := calls
+
+		if _, err := c.GetLyrics(
+			context.Background(),
+			"The Beatles",
+			"Yesterday",
+			"Help!",
+			125,
+		); err != nil {
+			t.Fatalf("GetLyrics: %v", err)
+		}
+
+		if calls != before {
+			t.Errorf("expected cache hit (no new HTTP call), calls went %d → %d", before, calls)
+		}
+	})
+
+	t.Run("404 maps to ErrLyricsNotFound", func(t *testing.T) {
+		_, err := c.GetLyrics(context.Background(), "Nobody", "Missing", "", 0)
+		if !errors.Is(err, ErrLyricsNotFound) {
+			t.Errorf("expected ErrLyricsNotFound, got %v", err)
+		}
+	})
+
+	t.Run("missing artist/title short-circuits without a request", func(t *testing.T) {
+		before := calls
+
+		if _, err := c.GetLyrics(
+			context.Background(),
+			"",
+			"Yesterday",
+			"",
+			0,
+		); !errors.Is(
+			err,
+			ErrLyricsNotFound,
+		) {
+			t.Errorf("expected ErrLyricsNotFound, got %v", err)
+		}
+
+		if calls != before {
+			t.Error("expected no HTTP call for empty artist")
+		}
+	})
+}