Add magnet link to metadata on fly from BTIH rather than torrent file

We currently add magnet links by parsing the torrent file. It looks like that is unneccessary, and can build direct from the BTIH (which is a metadata field on the _archive.torrent.xml file)

Start by locating the code that currently parses the torrent file to add magnet links, then inspect how BTIH is exposed in _archive.torrent.xml. The work is done when magnet links are built directly from that BTIH metadata without parsing the torrent file; verify the existing magnet-link behavior remains intact.

Written by the indexing model from the issue text.