🌐 feat: add configurable USD exchange-rate support across backend & frontend

Backend
- setting/payment.go: introduce default `USDExchangeRate` (7.3)
- model/option.go:
  • inject `USDExchangeRate` into `InitOptionMap`
  • persist & sync value in `updateOptionMap`
- controller/misc.go: expose `usd_exchange_rate` via `/api/status`

Frontend
- OperationSetting.js & SettingsGeneral.js:
  • extend state/inputs with `USDExchangeRate`
  • add form field “美元汇率 (non-top-up rate, pricing only)”
- ModelPricing.js already consumes `status.usd_exchange_rate`; no change needed

API
- Administrators can update the rate via `PUT /api/option` (key: `USDExchangeRate`)
- All clients receive the latest rate through `GET /api/status`

This closes the end-to-end flow for displaying model prices in both USD and CNY based on a configurable exchange rate.

web/src/components/settings/OperationSetting.js

@@ -19,6 +19,7 @@ const OperationSetting = () => {
     TopUpLink: '',
     'general_setting.docs_link': '',
     QuotaPerUnit: 0,
+    USDExchangeRate: 0,
     RetryTimes: 0,
     DisplayInCurrencyEnabled: false,
     DisplayTokenStatEnabled: false,