Add _seed_default_station() helper to /api/setup/seed endpoint that
creates the ST-DEFAULT station and assigns all active recipes to it,
preserving existing behaviour after migration 002 runs on live DBs.
Helper is idempotent and is called on both the normal seed path and
the early-return path (when demo data already exists).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
