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

main.go

@@ -78,7 +78,7 @@ func main() {
 		logger.Fatalf("❌ Failed to initialize database: %v", err)
 	}
 	defer st.Close()
-	backtest.UseDatabase(st.DB())
+	backtest.UseDatabaseWithType(st.DB(), st.DBType() == store.DBTypePostgres)
 
 	// Initialize installation ID for experience improvement (anonymous statistics)
 	initInstallationID(st)