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>

backend/api/sync_bookings.py

@@ -309,7 +309,9 @@ def run_bookings_data_sync(
                 return row.config_value
             return None
 
-        creds = {
+        # Central Settings service first, app-local config fallback
+        from services.central_settings import get_newbook_credentials_sync
+        creds = get_newbook_credentials_sync() or {
             'api_key': get_config('newbook_api_key'),
             'username': get_config('newbook_username'),
             'password': get_config('newbook_password'),