feat: 算力引擎 Go 后端核心（new-api）

- 统一 OpenAI 兼容 /v1 中继 + 渠道/额度/令牌/流水
- 各领域包：relay 模型网关、model 数据层、controller 管理 API

service/epay.go

@@ -0,0 +1,13 @@
+package service
+
+import (
+	"github.com/QuantumNous/new-api/setting/operation_setting"
+	"github.com/QuantumNous/new-api/setting/system_setting"
+)
+
+func GetCallbackAddress() string {
+	if operation_setting.CustomCallbackAddress == "" {
+		return system_setting.ServerAddress
+	}
+	return operation_setting.CustomCallbackAddress
+}