fix: 支持零价系统渠道模型分发 (#303)

* docs(spec): 模型计费 - 记录零价免费模型设计

* fix(billing): 模型定价 - 支持零价免费模型

* fix(config): 系统模型目录 - 支持零价模型

backend/internal/repository/finance.go

@@ -615,7 +615,7 @@ func (r *Repository) SettleBillingOrder(id string, providerRequestID string) err
 		if order.Status == model.BillingStatusRefunded {
 			return errors.New("billing order already refunded")
 		}
-		if order.BillingMode == "token" {
+		if order.BillingMode == "token" && !zeroPricedTokenOrder(order) {
 			usage, err := billingUsage(tx, id)
 			if err != nil {
 				return err
@@ -717,6 +717,13 @@ func (r *Repository) SettleBillingOrder(id string, providerRequestID string) err
 	})
 }
 
+func zeroPricedTokenOrder(order model.BillingOrder) bool {
+	return order.BillingMode == "token" &&
+		order.InputTokenPriceMicrocredits == 0 &&
+		order.OutputTokenPriceMicrocredits == 0 &&
+		order.CachedTokenPriceMicrocredits == 0
+}
+
 func (r *Repository) RefundBillingOrder(id string, errorText string) error {
 	return r.db.Transaction(func(tx *gorm.DB) error {
 		var order model.BillingOrder