fix: backtest module PostgreSQL compatibility and bug fixes

- Fix PostgreSQL placeholder conversion (? to $1, $2...) in all SQL queries
- Fix int4 overflow for timestamp columns (ALTER to BIGINT)
- Fix notional calculation bug in position Close() using proportional entry
- Fix potential panic in DecisionTimestamp with bounds check
- Fix nil pointer dereference in sliceUpTo with defensive checks
- Fix race condition in releaseLock using sync.Once
- Fix UnrealizedPnLPct always 0 in convertPositions
- Improve Sharpe ratio calculation with proper negative return handling

backtest/datafeed.go

@@ -124,11 +124,23 @@ func (df *DataFeed) DecisionBarCount() int {
 }
 
 func (df *DataFeed) DecisionTimestamp(index int) int64 {
+	// Bounds check to prevent panic
+	if index < 0 || index >= len(df.decisionTimes) {
+		return 0
+	}
 	return df.decisionTimes[index]
 }
 
 func (df *DataFeed) sliceUpTo(symbol, tf string, ts int64) []market.Kline {
-	series := df.symbolSeries[symbol].byTF[tf]
+	// Nil checks to prevent panic
+	ss, ok := df.symbolSeries[symbol]
+	if !ok || ss == nil {
+		return nil
+	}
+	series, ok := ss.byTF[tf]
+	if !ok || series == nil {
+		return nil
+	}
 	idx := sort.Search(len(series.closeTimes), func(i int) bool {
 		return series.closeTimes[i] > ts
 	})