feat: enhance backtest with real-time positions, P&L fixes, and strategy integration

- Add real-time position display with unrealized P&L during backtest
- Fix P&L calculation by tracking accumulated opening fees
- Add strategy coin source resolution (AI500, OI Top, mixed)
- Infer AI provider from model name for better compatibility
- Cap position size to available margin to prevent insufficient cash errors
- Fix trade markers on K-line chart (long/short instead of buy/sell)
- Add QuantData and OI ranking to backtest decision context

config/config.go

@@ -35,7 +35,7 @@ func Init() {
 	cfg := &Config{
 		APIServerPort:         8080,
 		RegistrationEnabled:   true,
-		MaxUsers:              1,    // Default: only 1 user allowed
+		MaxUsers:              5,    // Default: only 1 user allowed
 		ExperienceImprovement: true, // Default: enabled to help improve the product
 	}