NewBook credentials from central Settings service

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>

docker-compose.yml

@@ -7,10 +7,8 @@ services:
       - DATABASE_URL=${DATABASE_URL}
       - CENTRAL_AUTH_SECRET=${CENTRAL_AUTH_SECRET}
       - APP_SLUG=forecasting
-      - NEWBOOK_API_KEY=${NEWBOOK_API_KEY:-}
-      - NEWBOOK_USERNAME=${NEWBOOK_USERNAME:-}
-      - NEWBOOK_PASSWORD=${NEWBOOK_PASSWORD:-}
-      - NEWBOOK_REGION=${NEWBOOK_REGION:-AU}
+      - SETTINGS_URL=${SETTINGS_URL:-}
+      - SETTINGS_SECRET=${SETTINGS_SECRET:-}
       - RESOS_API_KEY=${RESOS_API_KEY:-}
       - ANTHROPIC_API_KEY=${ANTHROPIC_API_KEY:-}
     volumes: