GitLab

- Fix webhook URLs: /payment/webhook/<provider> not /payment/<provider>/notify
- Remove notifyUrl/returnUrl from provider config tables (auto-generated by UI)
- Adjust wxpay publicKeyId/certSerial to match frontend optional marking

- Add docs/PAYMENT.md and docs/PAYMENT_CN.md with full payment setup guide
- Mark Sub2ApiPay as deprecated in ecosystem tables (payment is now built-in)
- Add built-in payment system to features list in all 3 READMEs

Embedded pages (purchase subscription, custom pages) now receive the
current user locale through a `lang` URL parameter, allowing iframe
content to match the user's language preference.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Move ADMIN_PAYMENT_INTEGRATION_API.md → docs/ADMIN_PAYMENT_INTEGRATION_API.md
- Update README.md reference path
- Add payment integration doc download link in admin settings UI (Purchase section)
- Add i18n keys: integrationDoc / integrationDocHint (zh + en)

主要改动：
- 固定 Go 1.25.5 与 CI 校验并更新扫描流程
- 升级 quic-go、x/crypto、req 等依赖并通过 govulncheck
- 强化 JWT 校验、TLS 配置与 xlsx 动态加载
- 新增审计豁免清单与校验脚本