fix: 永久授权结账币种改为 CNY

与 pancake 永久商品 onetime+CNY 链接对齐，包年/永久均为 CNY。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/service/payment/waffo.go

@@ -41,12 +41,7 @@ func LoadWaffoConfig() WaffoConfig {
 	}
 	currencyPerpetual := strings.TrimSpace(os.Getenv("WAFFO_CURRENCY_PERPETUAL"))
 	if currencyPerpetual == "" {
-		// 永久默认 USD（与 pancake 商品一致）；可用环境变量覆盖。 // Logan.Li
-		if currency == "CNY" {
-			currencyPerpetual = "USD"
-		} else {
-			currencyPerpetual = currency
-		}
+		currencyPerpetual = currency
 	}
 	defaultProduct := strings.TrimSpace(os.Getenv("WAFFO_PRODUCT_ID"))
 	yearly := strings.TrimSpace(os.Getenv("WAFFO_PRODUCT_ID_YEARLY"))