Both import_one and import_season_pack_file deleted the existing
episode_file row and unlinked the on-disk file *before* link_or_copy_file
placed the new one. If the link/copy (or the free-space check) then
failed, the original content and its DB row were already gone with
nothing to fall back to.
Now the stale file is renamed to a sibling (freeing dest for the
hardlink fast path, same as before) and the DB row is left alone. Only
after the replacement is confirmed on disk are the file and the
old row cleaned up; any failure in between restores the original file
before returning the error.
