- Add active flag to Player model with idempotent ALTER TABLE migration
- Filter inactive players from main attendance table and API
- New /admin page with date generation (preview + bulk add) and player management (add/deactivate/reactivate)
- New API endpoints: GET/POST /api/players, PATCH /api/players/<id>
- Add Admin nav link on main page
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
