Mirror the pegasus exporter pattern: collect each ROM's media into a
canonical asset-kind dict, then either copy the files under
<platform>/assets/<subdir>/ for local exports or build absolute URLs
from request.base_url for remote exports. Exclude the generated assets/
directory from filesystem scans.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>