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/user.go

@@ -18,12 +18,6 @@ import (
 
 const UserNameMaxLength = 20
 
-var (
-	ErrDatabase              = errors.New("database error")
-	ErrInvalidCredentials    = errors.New("invalid credentials")
-	ErrUserEmptyCredentials  = errors.New("empty credentials")
-)
-
 // User if you add sensitive fields, don't forget to clean them in setupLogin function.
 // Otherwise, the sensitive information will be saved on local storage in plain text!
 type User struct {
@@ -766,16 +760,20 @@ func IsAdmin(userId int) bool {
 //	return user.Status == common.UserStatusEnabled, nil
 //}
 
-func ValidateAccessToken(token string) (user *User) {
+func ValidateAccessToken(token string) (*User, error) {
 	if token == "" {
-		return nil
+		return nil, nil
 	}
 	token = strings.Replace(token, "Bearer ", "", 1)
-	user = &User{}
-	if DB.Where("access_token = ?", token).First(user).RowsAffected == 1 {
-		return user
+	user := &User{}
+	err := DB.Where("access_token = ?", token).First(user).Error
+	if err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("%w: %v", ErrDatabase, err)
 	}
-	return nil
+	return user, nil
 }
 
 // GetUserQuota gets quota from Redis first, falls back to DB if needed