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_test.go

@@ -37,14 +37,14 @@ func TestBuildCheckoutURL_YearlyOnetimeCNY(t *testing.T) {
 	}
 }
 
-func TestBuildCheckoutURL_PerpetualOnetimeUSD(t *testing.T) {
+func TestBuildCheckoutURL_PerpetualOnetimeCNY(t *testing.T) {
 	cfg := WaffoConfig{
 		StoreSlug:          "opsxshell-pwsi6m4n",
 		ProductIDYearly:    "PROD_1BIezCi932IbgCzRIV8hcJ",
 		ProductIDPerpetual: "PROD_0pcnzzT4nalEkkcdyIUoD5",
 		Currency:           "CNY",
 		CurrencyYearly:     "CNY",
-		CurrencyPerpetual:  "USD",
+		CurrencyPerpetual:  "CNY",
 		Environment:        "test",
 		SuccessURL:         "opsxshell://pro/activated",
 	}
@@ -63,7 +63,7 @@ func TestBuildCheckoutURL_PerpetualOnetimeUSD(t *testing.T) {
 	if q.Get("type") != "onetime" {
 		t.Fatalf("type=%s want onetime", q.Get("type"))
 	}
-	if q.Get("currency") != "USD" {
-		t.Fatalf("currency=%s want USD", q.Get("currency"))
+	if q.Get("currency") != "CNY" {
+		t.Fatalf("currency=%s want CNY", q.Get("currency"))
 	}
 }