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/api/admin/payment.ts

@@ -23,6 +23,7 @@ export interface AdminPaymentConfig {
   max_pending_orders: number
   enabled_payment_types: string[]
   balance_disabled: boolean
+  balance_recharge_multiplier: number
   load_balance_strategy: string
   product_name_prefix: string
   product_name_suffix: string
@@ -40,6 +41,7 @@ export interface UpdatePaymentConfigRequest {
   max_pending_orders?: number
   enabled_payment_types?: string[]
   balance_disabled?: boolean
+  balance_recharge_multiplier?: number
   load_balance_strategy?: string
   product_name_prefix?: string
   product_name_suffix?: string

frontend/src/api/admin/settings.ts

@@ -125,6 +125,7 @@ export interface SystemSettings {
   payment_max_pending_orders: number
   payment_enabled_types: string[]
   payment_balance_disabled: boolean
+  payment_balance_recharge_multiplier: number
   payment_load_balance_strategy: string
   payment_product_name_prefix: string
   payment_product_name_suffix: string
@@ -231,6 +232,7 @@ export interface UpdateSettingsRequest {
   payment_max_pending_orders?: number
   payment_enabled_types?: string[]
   payment_balance_disabled?: boolean
+  payment_balance_recharge_multiplier?: number
   payment_load_balance_strategy?: string
   payment_product_name_prefix?: string
   payment_product_name_suffix?: string