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/artist_credit.sql

@@ -26,3 +26,9 @@ WHERE id = ?;
 
 -- name: DeleteAllArtistCredits :exec
 DELETE FROM artist_credit;
+
+-- name: CountArtistCreditReferences :one
+SELECT
+  (SELECT COUNT(*) FROM recordings WHERE artist_credit_id = ?1) +
+  (SELECT COUNT(*) FROM release_groups WHERE album_artist_credit_id = ?1)
+AS total;