fix: harden token auth error handling to prevent info leakage

- Create model/errors.go to centralize all sentinel errors
- ValidateAccessToken now returns error to distinguish DB failures
- ValidateUserToken uses unified ErrTokenInvalid for all auth failures
  (expired/exhausted/disabled/not-found) to prevent token enumeration
- authHelper and TokenAuthReadOnly use i18n messages instead of
  hardcoded Chinese strings
- All err.Error() removed from user-facing responses; DB errors logged
  server-side and return generic "contact admin" message (HTTP 500)
- Migrate ErrRedeemFailed, ErrTwoFANotEnabled to model/errors.go

model/errors.go

@@ -0,0 +1,26 @@
+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")
+)
+
+// 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")