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.
