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: