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/release_groups.sql

@@ -111,6 +111,9 @@ LEFT JOIN (
 WHERE aca.artist_id = ?
 ORDER BY rg.name;
 
+-- name: CountReleaseGroupRecordings :one
+SELECT COUNT(*) FROM release_group_recordings WHERE release_group_id = ?;
+
 -- name: GetAlbumsByArtistByLibrary :many
 SELECT
     rg.id,