feat(api): add GET/PUT /api/me/listenbrainz endpoints

Exposes user ListenBrainz config (token_set bool, enabled, last_scrobbled_at)
via write-only token semantics; enforces no-enable-without-token at the API layer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/library_fixtures_test.go

@@ -19,7 +19,7 @@ import (
 func truncateLibrary(t *testing.T, pool *pgxpool.Pool) {
 	t.Helper()
 	if _, err := pool.Exec(context.Background(),
-		"TRUNCATE tracks, albums, artists RESTART IDENTITY CASCADE"); err != nil {
+		"TRUNCATE tracks, albums, artists, scrobble_queue RESTART IDENTITY CASCADE"); err != nil {
 		t.Fatalf("truncate: %v", err)
 	}
 }