New POST /backup/download (admin-only) tars the whole jsondb directory
(all servers/clients/users/settings) and streams it back as a file
download. If a passphrase is given, the archive is encrypted first
(AES-256-GCM, scrypt-derived key, backup/encrypt.go) - a small
self-contained format, not gpg/OpenPGP-compatible, to avoid shelling
out to an external binary or adding a PGP dependency.
Deliberately does NOT upload anywhere automatically (e.g. to
Nextcloud) - the archive only ever leaves the server as this one HTTP
response to the requesting admin, who is responsible for storing it
themselves. New "Download Backup" button + passphrase modal on the
Global Settings page.
