Add metadata.pegasus.txt export alongside the existing gamelist.xml
export. Restructure the export system: rename the gamelist endpoint to
a general-purpose export endpoint (`/api/export/`) with sub-routes for
each format (`/gamelist-xml`, `/pegasus`). Move config from flat
`scan.export_gamelist` to nested `scan.export.gamelist_xml` and
`scan.export.pegasus` for auto-export on scan.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
