feat: Move 0DTE backtesting notebook and update algorithm (#620)

- Use bid-ask midpoint for Greeks, bid/ask for orders
- Add early assignment logic for realistic backtesting (define early assignment if the underlying price drops below 99.5% of the short strike, minus the premium received)
- Fix 0DTE options holding overnight in backtesting
    - Add expiration check at end of trading day

Author: idsts2670