Remove the level input from the boss defeat modal since the app doesn't
track levels elsewhere. Team selection is now just checkboxes without
requiring level entry.
- Remove level input UI from BossDefeatModal.tsx
- Add alembic migration to make boss_result_team.level nullable
- Update model and schemas to make level optional (defaults to null)
- Conditionally render level in boss result display
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
