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/recordings.sql

@@ -34,3 +34,6 @@ DELETE FROM recordings;
 -- name: GetAllRecordings :many
 SELECT * FROM recordings
 ORDER BY name;
+
+-- name: CountRecordingsByArtistCredit :one
+SELECT COUNT(*) FROM recordings WHERE artist_credit_id = ?;