When config_embed_metadata is enabled with config_binariesdir or
config_kepubifypath set, do_download_file stages a copy of every
downloaded book under get_temp_dir() but never removes it. A bulk
OPDS or Kobo sync can fill the host filesystem in minutes; the
effect is amplified for comic formats (CBZ/CBR), where each
staged file can run to hundreds of MB or several GB. Once the
disk fills, downloads silently 404 and a container restart does
not recover the space.
Add an after_this_request hook that removes the staged copy after
the response is sent, gated on filename == get_temp_dir() so the
non-embed and gdrive-direct paths are untouched.
