feat(16-02): add go-flac dependencies and implement FLAC tag writer

- Add go-flac/go-flac/v2, flacvorbis/v2, flacpicture/v2 dependencies
- Create tagwriter.go with TagChanges type, field constants, format detection, MIME detection
- Create flac.go with writeFlacTags using Vorbis Comments + PICTURE blocks + AtomicWrite
- Implement replaceVorbisComment helper for case-insensitive field replacement
- Handle cover art add/replace/clear via PICTURE metadata blocks

backend/database/sql/sqlcgen/recordings.sql.go

@@ -10,6 +10,17 @@ import (
 	"database/sql"
 )
 
+const countRecordingsByArtistCredit = `-- name: CountRecordingsByArtistCredit :one
+SELECT COUNT(*) FROM recordings WHERE artist_credit_id = ?
+`
+
+func (q *Queries) CountRecordingsByArtistCredit(ctx context.Context, artistCreditID int64) (int64, error) {
+	row := q.db.QueryRowContext(ctx, countRecordingsByArtistCredit, artistCreditID)
+	var count int64
+	err := row.Scan(&count)
+	return count, err
+}
+
 const createRecording = `-- name: CreateRecording :one
 INSERT INTO recordings (name, artist_credit_id) VALUES (?, ?)
 RETURNING id, name, artist_credit_id, track_number, disc_number, year, genre, composer, lyrics, comment