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_test.go

@@ -11,7 +11,7 @@ func TestBuildCheckoutURL_YearlySubscription(t *testing.T) {
 		StoreSlug:          "opsxshell-pwsi6m4n",
 		ProductIDYearly:    "PROD_2WmkUVoYWt0NN8OVEyC6n3",
 		ProductIDPerpetual: "PROD_0pcnzzT4nalEkkcdyIUoD5",
-		Currency:           "CNY",
+		Currency:           "USD",
 		Environment:        "test",
 		SuccessURL:         "opsxshell://pro/activated",
 	}
@@ -30,7 +30,7 @@ func TestBuildCheckoutURL_YearlySubscription(t *testing.T) {
 	if q.Get("type") != "subscription" {
 		t.Fatalf("type=%s want subscription", q.Get("type"))
 	}
-	if q.Get("currency") != "CNY" {
+	if q.Get("currency") != "USD" {
 		t.Fatalf("currency=%s", q.Get("currency"))
 	}
 	if q.Get("test") != "true" {
@@ -43,7 +43,7 @@ func TestBuildCheckoutURL_PerpetualOnetime(t *testing.T) {
 		StoreSlug:          "opsxshell-pwsi6m4n",
 		ProductIDYearly:    "PROD_2WmkUVoYWt0NN8OVEyC6n3",
 		ProductIDPerpetual: "PROD_0pcnzzT4nalEkkcdyIUoD5",
-		Currency:           "CNY",
+		Currency:           "USD",
 		Environment:        "test",
 		SuccessURL:         "opsxshell://pro/activated",
 	}