Change _remove_episode_from_missing_list to set is_downloaded=True
and populate file_path via EpisodeService.mark_downloaded, instead of
deleting the Episode row. Preserves download history so queries can
distinguish series with downloaded episodes from completely unwatched
series.
- Pass serie_folder to construct file_path
- Look up series_id via AnimeSeriesService.get_by_key
- Update tests to mock mark_downloaded path
- Document episode lifecycle in docs/DEVELOPMENT.md
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>