feat(payment): 订阅 CNY 换算改为独立汇率配置的显式 opt-in

- 新增 SUBSCRIPTION_USD_TO_CNY_RATE 配置（1 USD = X CNY，默认 0=关闭），
  替代复用 balance_recharge_multiplier 的隐式换算，促销倍率与订阅定价解耦
- 未配置汇率时订阅保持 price 直付的存量行为，存量部署升级零影响
- 前端确认页/原价/手续费/方式限额与后端换算条件严格镜像（rate>0 且币种为 CNY）
- 管理后台新增汇率配置输入（zh/en 文案），checkout-info 透出 subscription_usd_to_cny_rate
- 单测锁定：汇率未配置时不换算、换算使用汇率而非余额倍率、余额订单不受影响、返利仍按 USD price

frontend/src/api/admin/payment.ts

@@ -24,6 +24,7 @@ export interface AdminPaymentConfig {
   enabled_payment_types: string[]
   balance_disabled: boolean
   balance_recharge_multiplier: number
+  subscription_usd_to_cny_rate: number
   load_balance_strategy: string
   product_name_prefix: string
   product_name_suffix: string
@@ -42,6 +43,7 @@ export interface UpdatePaymentConfigRequest {
   enabled_payment_types?: string[]
   balance_disabled?: boolean
   balance_recharge_multiplier?: number
+  subscription_usd_to_cny_rate?: number
   load_balance_strategy?: string
   product_name_prefix?: string
   product_name_suffix?: string

frontend/src/api/admin/settings.ts

@@ -589,6 +589,7 @@ export interface SystemSettings {
   payment_enabled_types: string[];
   payment_balance_disabled: boolean;
   payment_balance_recharge_multiplier: number;
+  payment_subscription_usd_to_cny_rate: number;
   payment_recharge_fee_rate: number;
   payment_load_balance_strategy: string;
   payment_product_name_prefix: string;
@@ -860,6 +861,7 @@ export interface UpdateSettingsRequest {
   payment_enabled_types?: string[];
   payment_balance_disabled?: boolean;
   payment_balance_recharge_multiplier?: number;
+  payment_subscription_usd_to_cny_rate?: number;
   payment_recharge_fee_rate?: number;
   payment_load_balance_strategy?: string;
   payment_product_name_prefix?: string;