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/redemption.go

@@ -11,9 +11,6 @@ import (
 	"gorm.io/gorm"
 )
 
-// ErrRedeemFailed is returned when redemption fails due to database error
-var ErrRedeemFailed = errors.New("redeem.failed")
-
 type Redemption struct {
 	Id           int            `json:"id"`
 	UserId       int            `json:"user_id"`