chore: Waffo 网关默认币种改为 USD

与 pancake 商品链接 currency=USD 对齐。

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:    "CNY",
+			Currency:    "USD",
 			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 = "CNY"
+		existing.Currency = "USD"
 		existing.Priority = 0
 		if err := db.Save(&existing).Error; err != nil {
 			return err