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/en.yaml

@@ -23,6 +23,16 @@ common.already_exists: "Already exists"
 common.name_cannot_be_empty: "Name cannot be empty"
 common.batch_too_many: "Too many items in batch request, maximum is {{.Max}}"
 
+# Auth middleware messages
+auth.not_logged_in: "Unauthorized, not logged in and no access token provided"
+auth.access_token_invalid: "Unauthorized, invalid access token"
+auth.user_info_invalid: "Unauthorized, invalid user info"
+auth.user_id_not_provided: "Unauthorized, New-Api-User header not provided"
+auth.user_id_format_error: "Unauthorized, New-Api-User header format error"
+auth.user_id_mismatch: "Unauthorized, New-Api-User does not match logged in user"
+auth.user_banned: "User has been banned"
+auth.insufficient_privilege: "Unauthorized, insufficient privileges"
+
 # Token messages
 token.name_too_long: "Token name is too long"
 token.quota_negative: "Quota value cannot be negative"