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

model/errors.go

@@ -0,0 +1,30 @@
+package model
+
+import "errors"
+
+// Common errors
+var (
+	ErrDatabase = errors.New("database error")
+)
+
+// User auth errors
+var (
+	ErrInvalidCredentials   = errors.New("invalid credentials")
+	ErrUserEmptyCredentials = errors.New("empty credentials")
+	ErrEmailAlreadyTaken    = errors.New("email already taken")
+	ErrEmailNotFound        = errors.New("email not found")
+	ErrEmailAmbiguous       = errors.New("email matches multiple users")
+)
+
+// Token auth errors
+var (
+	ErrTokenNotProvided = errors.New("token not provided")
+	ErrTokenInvalid     = errors.New("token invalid")
+)
+
+// Redemption errors
+var ErrRedeemFailed = errors.New("redeem.failed")
+
+// 2FA errors
+var ErrTwoFANotEnabled = errors.New("2fa not enabled")
+var ErrTwoFAAlreadyEnabled = errors.New("2fa already enabled")