Serves the dashboard's own precompiled app.css / app.js from
priv/static/assets, so the host needs no Plug.Static configuration and the
bundles can't collide with the host's own assets.
forwarded by bloccs_dashboard/2 at <path>/assets, so the filename arrives
as the remaining path_info segment. A long cache-control is safe because
the filenames are content-stable per release.