fix: normalize old backtest field names in VBT dashboard API
Old files used pnl_pct (not total_return_pct), max_dd (decimal,
not max_drawdown_pct %), num_periods (not n_bars), no profit_factor.
Added _normalize_vbt_fields() that:
- Maps pnl_pct/ann_return_pct → total_return_pct
- Converts max_dd (decimal) → max_drawdown_pct (percentage)
- Maps num_periods → n_bars
- Computes profit_factor from trades (gross_wins / gross_losses)
- Computes win_rate from trades if missing
Both /api/vbt/results and /api/vbt/result/{filename} now normalise.
Verified: old Cartea-Jaimungal file now shows ret=0.82%, pf=1.18, bars=720
ramseshk
623345c4d7