feat: add 'Ask' quality option, language auto-select, and m4a audio remux

- Add 'ask' option to audio/video quality settings (stored as '' in DB)
- Auto-select when both media type AND quality are set; show filtered
  format picker when quality is Ask
- Auto-skip language picker when user has a matching language preference
- Remux audio-only downloads to m4a instead of webm for Telegram compat
- Update README to document the new flow and m4a audio output

internal/repo/store.go

@@ -65,7 +65,7 @@ func (s *Store) GetOrCreateUser(chatID int64) (int64, error) {
 
 func (s *Store) GetOrCreatePreferences(userID int64) (*domain.UserPreferences, error) {
 	if _, err := s.db.Exec(
-		"INSERT OR IGNORE INTO user_preferences (user_id, default_media_type) VALUES (?, 'ask')",
+		"INSERT OR IGNORE INTO user_preferences (user_id, default_media_type, default_audio_format, default_video_format) VALUES (?, 'ask', '', '')",
 		userID,
 	); err != nil {
 		return nil, err