GitLab

Tests (1033 new lines, 100% coverage on modified functions):
- amount.go: YuanToFen/FenToYuan with precision edge cases
- wxpay: mapWxState, wxSV, formatPEM, NewWxpay validation
- alipay: isTradeNotExist, NewAlipay validation
- webhook: writeSuccessResponse (wxpay JSON, stripe empty, others text)
- config: validateProviderRequest, isSensitiveConfigField, joinTypes
- fulfillment: resolveRedeemAction idempotency logic

Business logic changes:
- Allow empty supported_types on provider instances
- Block removing payment types when instance has pending orders
- Extract resolveRedeemAction as testable pure function

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