_extract_audio wrote transcoded audio to a .tmp temp path, so ffmpeg
couldn't guess a muxer from the filename and aborted on every video
episode whose source was still reachable. Pass -f mp3 explicitly instead
of relying on the extension.
Separately, a permanently 404'd episode was retried on every startup and
every 6-hour poll forever, since nothing remembered past failures. Track
failed downloads per show folder with a 7-day backoff before retrying.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>