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

frontend/src/types/payment.ts

@@ -32,6 +32,7 @@ export interface PaymentConfig {
   max_pending_orders: number
   order_timeout_minutes: number
   balance_disabled: boolean
+  balance_recharge_multiplier: number
   enabled_payment_types: PaymentType[]
   help_image_url: string
   help_text: string
@@ -62,6 +63,7 @@ export interface CheckoutInfoResponse {
   global_max: number
   plans: SubscriptionPlan[]
   balance_disabled: boolean
+  balance_recharge_multiplier: number
   help_text: string
   help_image_url: string
   stripe_publishable_key: string
@@ -155,6 +157,7 @@ export interface CreateOrderRequest {
 
 export interface CreateOrderResult {
   order_id: number
+  amount: number
   pay_url?: string
   qr_code?: string
   client_secret?: string