fix: Waffo webhook 支持 GET 探测，避免浏览器打开 404

仪表盘/浏览器用 GET 校验 URL 时返回 ready；支付事件仍走 POST。

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

backend/internal/service/payment/waffo.go

@@ -31,7 +31,7 @@ func LoadWaffoConfig() WaffoConfig {
 	}
 	currency := strings.TrimSpace(os.Getenv("WAFFO_CURRENCY"))
 	if currency == "" {
-		currency = "CNY"
+		currency = "USD"
 	}
 	defaultProduct := strings.TrimSpace(os.Getenv("WAFFO_PRODUCT_ID"))
 	yearly := strings.TrimSpace(os.Getenv("WAFFO_PRODUCT_ID_YEARLY"))