refactor: rename Format to Quality, add container Audio/Video sub-pickers

- Rename 'Format' button to 'Quality' in settings (audio_quality/video_quality)
- Add DefaultAudioContainer and DefaultVideoContainer to UserPreferences
- Container settings now has Audio/Video sub-pickers (mp3/m4a/opus vs mp4/mkv/webm)
- Add migration 003 for new container columns
- Update repo layer and playlist flow for new fields
- Update back routing: back_quality, back_container

internal/repo/store.go

@@ -76,13 +76,13 @@ func (s *Store) GetOrCreatePreferences(userID int64) (*domain.UserPreferences, e
 func (s *Store) getPreferences(userID int64) (*domain.UserPreferences, error) {
 	row := s.db.QueryRow(`
 		SELECT default_media_type, default_audio_format, default_video_format,
-		       default_container, default_language
+		       default_audio_container, default_video_container, default_language
 		FROM user_preferences WHERE user_id = ?
 	`, userID)
 	var p domain.UserPreferences
 	var mediaType string
 	if err := row.Scan(&mediaType, &p.DefaultAudioFormat, &p.DefaultVideoFormat,
-		&p.DefaultContainer, &p.DefaultLanguage); err != nil {
+		&p.DefaultAudioContainer, &p.DefaultVideoContainer, &p.DefaultLanguage); err != nil {
 		return nil, err
 	}
 	switch mediaType {
@@ -110,10 +110,12 @@ func mediaTypeToString(mt domain.MediaType) string {
 func (s *Store) UpdatePreferences(userID int64, prefs *domain.UserPreferences) error {
 	_, err := s.db.Exec(`
 		UPDATE user_preferences SET default_media_type=?, default_audio_format=?,
-		       default_video_format=?, default_container=?, default_language=?
+		       default_video_format=?, default_audio_container=?,
+		       default_video_container=?, default_language=?
 		WHERE user_id=?
 	`, mediaTypeToString(prefs.DefaultMediaType), prefs.DefaultAudioFormat,
-		prefs.DefaultVideoFormat, prefs.DefaultContainer, prefs.DefaultLanguage, userID)
+		prefs.DefaultVideoFormat, prefs.DefaultAudioContainer,
+		prefs.DefaultVideoContainer, prefs.DefaultLanguage, userID)
 	return err
 }