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_refund.go

@@ -113,11 +113,7 @@ func (s *PaymentService) PrepareRefund(ctx context.Context, oid int64, amt float
 	if amt-o.Amount > amountToleranceCNY {
 		return nil, nil, infraerrors.BadRequest("REFUND_AMOUNT_EXCEEDED", "refund amount exceeds recharge")
 	}
-	// Full refund: use actual pay_amount for gateway (includes fees)
-	ga := amt
-	if math.Abs(amt-o.Amount) <= amountToleranceCNY {
-		ga = o.PayAmount
-	}
+	ga := calculateGatewayRefundAmount(o.Amount, o.PayAmount, amt)
 	rr := strings.TrimSpace(reason)
 	if rr == "" && o.RefundRequestReason != nil {
 		rr = *o.RefundRequestReason