feat: add candlestick chart with portfolio value line (BT-004)

- Add spot_open field to BacktestDailyPoint for complete OHLC data
- Replace line chart with candlestick chart showing price OHLC
- Add portfolio value line on secondary Y-axis
- Add _chart_options_from_dict for rendering job results
- Update both render_result and render_job_result to use new chart

app/backtesting/engine.py

@@ -94,6 +94,7 @@ class SyntheticBacktestEngine:
                 BacktestDailyPoint(
                     date=day.date,
                     spot_close=day.close,
+                    spot_open=day.open if day.open is not None else day.close,
                     spot_low=day.low if day.low is not None else day.close,
                     spot_high=day.high if day.high is not None else day.close,
                     underlying_value=underlying_value_close,