feat: mail service

gateway/TODO.md

@@ -1,10 +1,14 @@
 # TODOs
 
-## 1. Suggest User's Preferred Language when registering a new User
+## 1. Improve Preferred-Language Fallback after the Current Accept-Language Rollout
 
-Upon user's device/session registration flow, `preferred_language` value
-must be obtained via existing [geoip](../pkg/geoip) package by returned
-country.
-The derived value must be emitted as a valid BCP 47 language tag because
-`User Service` now validates that contract semantically on create.
-When geoip fails to return country by IP, fallback is `en`.
+The current auth-registration flow derives the preferred-language candidate
+from the public `Accept-Language` header and falls back to `en` when no
+supported tag is available.
+
+A later improvement may use the existing [geoip](../pkg/geoip) package as an
+additional fallback when `Accept-Language` is absent or unusable, but it must:
+
+- preserve the current public JSON DTOs
+- continue emitting a valid BCP 47 tag for `User Service`
+- keep `en` as the final safe fallback