feat: game lobby service

user/docs/runbook.md

@@ -58,16 +58,19 @@ Checks:
 - `preferred_language` is a valid BCP 47 tag
 - `time_zone` is a valid IANA time-zone name
 
-### race_name conflict
+### profile update rejected
 
 Symptoms:
 
-- profile update returns `409 conflict`
+- profile update returns `400 invalid_request` or `409 conflict`
 
 Checks:
 
-- desired race name is not already reserved under canonical uniqueness rules
+- submitted `display_name` passes `pkg/util/string.go:ValidateTypeName`; empty
+  values are accepted and reset the stored display name
 - user is not currently blocked by `profile_update_block`
+- `user_name` is immutable; any attempt to mutate it surfaces as
+  `409 conflict`
 
 ### declared-country sync rejected