feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

frontend/src/api/index.ts

@@ -14,6 +14,7 @@ export { keysAPI } from './keys'
 export { usageAPI } from './usage'
 export { userAPI } from './user'
 export { redeemAPI, type RedeemHistoryItem } from './redeem'
+export { paymentAPI } from './payment'
 export { userGroupsAPI } from './groups'
 export { totpAPI } from './totp'
 export { default as announcementsAPI } from './announcements'