spawn_torrent parses each torrent's file list (at add and restore) and
refuses an add whose files would write where a registered torrent's data
lives (NAUT_ERR_EXIST). Magnets are checked once metadata is known is
out of scope; restore skips the check. webui surfaces it as HTTP 409.
Mark #10 done.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>