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;

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

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 = ?;

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,