feat(waffo): Waffo payment gateway integration with configurable methods

- Add Waffo payment SDK integration (waffo-go v1.3.1)
- Backend: webhook handler, pay endpoint, order lock race-condition fix
- Settings: full Waffo config (API keys, sandbox/prod, currency, pay methods)
- Frontend: Waffo payment buttons in topup page, admin settings panel
- i18n: Waffo-related translations for en/fr/ja/ru/vi/zh-TW

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

common/constants.go

@@ -211,5 +211,6 @@ const (
 const (
 	TopUpStatusPending = "pending"
 	TopUpStatusSuccess = "success"
+	TopUpStatusFailed  = "failed"
 	TopUpStatusExpired = "expired"
 )