🌐 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/i18n/locales/en.json

@@ -1779,5 +1779,7 @@
   "将仅保留第一个密钥文件，其余文件将被移除，是否继续？": "Only the first key file will be retained, and the remaining files will be removed. Continue?",
   "自定义模型名称": "Custom model name",
   "启用全部密钥": "Enable all keys",
-  "以充值价格显示": "Show with recharge price"
+  "以充值价格显示": "Show with recharge price",
+  "美元汇率（非充值汇率，仅用于定价页面换算）": "USD exchange rate (not recharge rate, only used for pricing page conversion)",
+  "美元汇率": "USD exchange rate"
 }