feat(payment): balance recharge multiplier and refund amount separation

- Add balance_recharge_multiplier system setting (e.g. 1.2 = charge 100 get 120)
- Separate order_amount (credited balance) from pay_amount (actual payment)
- Refund calculates gateway amount proportionally from pay_amount
- Frontend shows both amounts in order details, payment status, refund dialog
- Admin settings UI for configuring recharge multiplier

backend/internal/service/payment_fulfillment.go

@@ -216,7 +216,11 @@ func (s *PaymentService) markCompleted(ctx context.Context, o *dbent.PaymentOrde
 	if err != nil {
 		return fmt.Errorf("mark completed: %w", err)
 	}
-	s.writeAuditLog(ctx, o.ID, auditAction, "system", map[string]any{"rechargeCode": o.RechargeCode, "amount": o.Amount})
+	s.writeAuditLog(ctx, o.ID, auditAction, "system", map[string]any{
+		"rechargeCode":   o.RechargeCode,
+		"creditedAmount": o.Amount,
+		"payAmount":      o.PayAmount,
+	})
 	return nil
 }