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/sqlcgen/artist_credit.sql.go

@@ -9,6 +9,20 @@ import (
 	"context"
 )
 
+const countArtistCreditReferences = `-- 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
+`
+
+func (q *Queries) CountArtistCreditReferences(ctx context.Context, artistCreditID int64) (int64, error) {
+	row := q.db.QueryRowContext(ctx, countArtistCreditReferences, artistCreditID)
+	var total int64
+	err := row.Scan(&total)
+	return total, err
+}
+
 const createArtistCredit = `-- name: CreateArtistCredit :one
 INSERT INTO artist_credit (text) VALUES (?)
 RETURNING id, text

backend/database/sql/sqlcgen/genres.sql.go

@@ -10,6 +10,17 @@ import (
 	"database/sql"
 )
 
+const countGenreReferences = `-- name: CountGenreReferences :one
+SELECT COUNT(*) FROM recording_genres WHERE genre_id = ?
+`
+
+func (q *Queries) CountGenreReferences(ctx context.Context, genreID int64) (int64, error) {
+	row := q.db.QueryRowContext(ctx, countGenreReferences, genreID)
+	var count int64
+	err := row.Scan(&count)
+	return count, err
+}
+
 const createRecordingGenre = `-- name: CreateRecordingGenre :exec
 INSERT OR IGNORE INTO recording_genres (recording_id, genre_id)
 VALUES (?, ?)
@@ -43,6 +54,15 @@ func (q *Queries) DeleteAllRecordingGenres(ctx context.Context) error {
 	return err
 }
 
+const deleteGenre = `-- name: DeleteGenre :exec
+DELETE FROM genres WHERE id = ?
+`
+
+func (q *Queries) DeleteGenre(ctx context.Context, id int64) error {
+	_, err := q.db.ExecContext(ctx, deleteGenre, id)
+	return err
+}
+
 const deleteRecordingGenres = `-- name: DeleteRecordingGenres :exec
 DELETE FROM recording_genres
 WHERE recording_id = ?

backend/database/sql/sqlcgen/recordings.sql.go

@@ -10,6 +10,17 @@ import (
 	"database/sql"
 )
 
+const countRecordingsByArtistCredit = `-- name: CountRecordingsByArtistCredit :one
+SELECT COUNT(*) FROM recordings WHERE artist_credit_id = ?
+`
+
+func (q *Queries) CountRecordingsByArtistCredit(ctx context.Context, artistCreditID int64) (int64, error) {
+	row := q.db.QueryRowContext(ctx, countRecordingsByArtistCredit, artistCreditID)
+	var count int64
+	err := row.Scan(&count)
+	return count, err
+}
+
 const createRecording = `-- name: CreateRecording :one
 INSERT INTO recordings (name, artist_credit_id) VALUES (?, ?)
 RETURNING id, name, artist_credit_id, track_number, disc_number, year, genre, composer, lyrics, comment

backend/database/sql/sqlcgen/release_groups.sql.go

@@ -10,6 +10,17 @@ import (
 	"database/sql"
 )
 
+const countReleaseGroupRecordings = `-- name: CountReleaseGroupRecordings :one
+SELECT COUNT(*) FROM release_group_recordings WHERE release_group_id = ?
+`
+
+func (q *Queries) CountReleaseGroupRecordings(ctx context.Context, releaseGroupID int64) (int64, error) {
+	row := q.db.QueryRowContext(ctx, countReleaseGroupRecordings, releaseGroupID)
+	var count int64
+	err := row.Scan(&count)
+	return count, err
+}
+
 const createReleaseGroup = `-- name: CreateReleaseGroup :one
 INSERT INTO release_groups (name) VALUES (?)
 RETURNING id, name, cover_art_id, album_artist_credit_id, year, total_tracks, total_discs