Reuse the existing nzbget chart as the usenet downloader. Newshosting
username/password come from the out-of-band `usenet-creds` Opaque secret
(same pattern as gluetun-wireguard), exposed as env and referenced in
nzbget.conf via ${NEWSHOSTING_USER}/${NEWSHOSTING_PASS} so no plaintext
provider password lands in the config file.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>