calibredb export writes <uuid>.jpg next to <uuid>.<format> by
default, leaving an unused cover image in /tmp/calibre_web for
every download that goes through do_calibre_export. The cover
file is never read by calibre-web (cover serving uses a separate
path), so it is pure waste of disk space and IO.
Pass --dont-save-cover, paired with the existing --dont-write-opf,
to skip the unwanted side-effect at the source. Verified that
none of the three do_calibre_export call sites (download via
do_download_file, email via tasks/mail.py, kepubify via
tasks/convert.py) read the cover sibling.
