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

i18n/locales/zh-TW.yaml

@@ -24,6 +24,16 @@ common.already_exists: "已存在"
 common.name_cannot_be_empty: "名稱不能為空"
 common.batch_too_many: "批次請求數量過多，最多 {{.Max}} 條"
 
+# Auth middleware messages
+auth.not_logged_in: "無權進行此操作，未登入且未提供 access token"
+auth.access_token_invalid: "無權進行此操作，access token 無效"
+auth.user_info_invalid: "無權進行此操作，使用者資訊無效"
+auth.user_id_not_provided: "無權進行此操作，未提供 New-Api-User"
+auth.user_id_format_error: "無權進行此操作，New-Api-User 格式錯誤"
+auth.user_id_mismatch: "無權進行此操作，New-Api-User 與登入使用者不匹配"
+auth.user_banned: "使用者已被封禁"
+auth.insufficient_privilege: "無權進行此操作，權限不足"
+
 # Token messages
 token.name_too_long: "令牌名稱過長"
 token.quota_negative: "額度值不能為負數"