feat(payment): add native USDT payment provider with live CNY rate display

[s258852s]

Summary

Adds a first-class usdt payment provider so EasyPay-protocol crypto gateways that settle in stablecoins (e.g. self-hosted BEpusdt) can be configured directly from the admin UI, instead of being shoehorned into the alipay/wxpay sub-types of the generic EasyPay provider.

Bundled with the provider is a small UX helper: a live CNY/USDT rate is shown beside the USDT method on the recharge page, so users know roughly how much USDT they will pay before being redirected to the gateway's checkout.

Motivation

Without a native provider, self-hosting a crypto gateway today requires either:

• Overloading the EasyPay alipay or wxpay channel — confusing UI, brittle signing semantics, and the order's payment_type does not match what the user actually paid in.
• Running an external re-signing bridge — extra moving part, extra failure mode.

A real usdt provider is ~170 lines that fits cleanly into the existing EasyPay helpers, with no new dependencies or protocol fork.

What's added

Backend

• payment/provider/usdt.go — new provider, builds EasyPay-style submit requests with the on-chain trade type sent natively (usdt.trc20 by default, configurable per instance). Sign/verify reuse the EasyPay MD5 helpers; only the wire-level type field and the snapshot/notification metadata differ from EasyPay.
• Provider key usdt registered in factory, types, webhook routes, webhook handler, validProviderKeys, sensitive-field list, and the pending-order-protected config field list.
• PID is snapshotted on order create and verified on notification, matching how alipay / wxpay / easypay already enforce merchant identity.
• GET /api/v1/payment/usdt/rate — cached (60s, singleflight, stale-on-error) CNY/USDT rate fetched from CoinGecko. Used by the recharge UI to preview the implied USDT amount.

Frontend

• USDT is selectable in admin provider management, with its own icon (assets/icons/usdt.svg) and brand color (#26A17B), wired through providerConfig.ts, the payment flow helpers, admin order tables/filters, i18n (zh + en) and types/payment.ts.
• PaymentMethodSelector accepts an optional usdtRate + usdtImpliedAmount. When USDT is among the visible methods, it renders an inline hint — Live rate 1 USDT ≈ X CNY · pay ~Y USDT — plus a one-line note that the checkout page amount is authoritative.
• PaymentView polls /payment/usdt/rate on mount and every 60s, passing the rate + entered amount to the selector. The line softly hides if the rate fetch fails, so the UI never blocks.

Compatibility

• No DB migration required — uses the existing payment_provider_instances JSON config.
• No breaking changes to existing providers; only additive.

Testing

• Verified end-to-end against a self-hosted BEpusdt instance: a real TRC20 payment (10.06 USDT for 68 CNY at a live CoinGecko rate of 6.76) was created, confirmed on-chain, notified back via webhook, and credited to the user balance.
• go build ./..., golangci-lint, vue-tsc --noEmit and the existing vitest critical suite all pass.

CLA

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