feat: mail service

user/docs/flows.md

@@ -28,8 +28,9 @@ Rules:
 - `registration_context` is create-only
 - existing users ignore the supplied registration context
 - blocked subjects return `blocked` rather than creating a user
-- the current rollout sends temporary `preferred_language="en"` from
-  authsession and forwards the public confirm `time_zone`
+- the current rollout sends the preferred-language candidate derived from
+  public `Accept-Language`, falls back to `en` when needed, and forwards the
+  public confirm `time_zone`
 
 Create side effects:
 

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