Scriptable equivalent of HyperKitty's web Download button for
backup-and-destroy loops. Runs the export through Debian's mailman-web
wrapper (HyperKitty lives in the web DB, separate from core), reproducing
the web view's Email.as_bytes() output.
- Handles the three archive states without writing partial/empty files:
populated (writes mbox), row-but-empty, and no row at all.
- For a missing row, best-effort core REST lookup distinguishes
archive_policy=never from 'nothing archived yet'.
- -z/--gzip matches the web .mbox.gz (deterministic header, mtime=0).
- Privilege-safe: web user writes a 0600 mkstemp file, root moves it.
- Address validated before interpolation into the embedded Python.
