feat: add payoff_ratio and recovery_factor metrics, bump to 0.3.2

Add two new risk/reward metrics to BacktestMetrics:
- payoff_ratio: avg winning return / avg losing return (absolute)
- recovery_factor: net profit / max drawdown in absolute terms
Computed in both StreamingMetrics::finalize() and PortfolioEngine.
Exposed via PyO3 with #[pyo3(get)] on PyBacktestMetrics.
Updated README with API reference and changelog.

python/raptorbt/__init__.py

@@ -43,7 +43,7 @@ from raptorbt._raptorbt import (
     rolling_max,
 )
 
-__version__ = "0.3.1"
+__version__ = "0.3.2"
 
 __all__ = [
     # Config classes