fix: 包年改为 Waffo 一次性商品并切 CNY

包年 SKU 换为 PROD_1BIezCi932IbgCzRIV8hcJ，checkout type 统一 onetime。

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

backend/database/seed.go

@@ -282,7 +282,7 @@ func ensureWaffoPaymentGateway(db *gorm.DB) error {
 			Gateway:     "waffo",
 			Enabled:     true,
 			SandboxMode: true,
-			Currency:    "USD",
+			Currency:    "CNY",
 			Priority:    0,
 		}
 		if err := db.Create(&gw).Error; err != nil {
@@ -291,7 +291,7 @@ func ensureWaffoPaymentGateway(db *gorm.DB) error {
 	} else {
 		existing.Enabled = true
 		existing.SandboxMode = true
-		existing.Currency = "USD"
+		existing.Currency = "CNY"
 		existing.Priority = 0
 		if err := db.Save(&existing).Error; err != nil {
 			return err
@@ -303,6 +303,6 @@ func ensureWaffoPaymentGateway(db *gorm.DB) error {
 		Update("priority", 100).Error; err != nil {
 		return err
 	}
-	log.Println("[初始化] 支付网关已优先 Waffo（包年 subscription / 永久 onetime）")
+	log.Println("[初始化] 支付网关已优先 Waffo（包年/永久均为 onetime）")
 	return nil
 }