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/queries/genres.sql

@@ -103,6 +103,12 @@ LEFT JOIN file_types ft ON af.file_type_id = ft.id
 WHERE g.name = ? AND af.library_id = ?
 ORDER BY r.name;
 
+-- name: CountGenreReferences :one
+SELECT COUNT(*) FROM recording_genres WHERE genre_id = ?;
+
+-- name: DeleteGenre :exec
+DELETE FROM genres WHERE id = ?;
+
 -- name: GetAllGenresWithCounts :many
 SELECT g.name, COUNT(rg.recording_id) AS track_count
 FROM genres g