feat: mail service

user/docs/runbook.md

@@ -84,8 +84,9 @@ Checks:
 
 - Keep `Auth / Session Service` and `User Service` aligned on the current
   `registration_context` shape.
-- During the current rollout, treat authsession-provided
-  `preferred_language="en"` as the active create-path contract.
+- During the current rollout, treat the authsession-provided
+  `preferred_language` derived from public `Accept-Language`, with fallback to
+  `en`, as the active create-path contract.
 - Gateway direct `user.*` self-service routing depends on the internal REST
   routes staying stable.
 - Do not roll out billing-driven entitlement mutations assuming another