Stack-wide NewBook config lives in the Settings app (LXC 116) and is
fetched live via SETTINGS_URL/SETTINGS_SECRET — same pattern as cashup,
room-planner and maintenance. App-local system_config credentials remain
as a fallback for standalone/dev use. The app's Settings → Newbook page
no longer edits credentials; it points to the central app and keeps
Test Connection.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>