feat: game lobby service

user/docs/flows.md

@@ -66,15 +66,16 @@ Rules:
 
 ### Profile update
 
-`UpdateMyProfile` changes only `race_name`.
+`UpdateMyProfile` changes only `display_name`.
 
 Rules:
 
-- preserve stored casing on success
-- enforce canonical reservation uniqueness
-- reject conflicts as `409 conflict`
+- validate the submitted value through `pkg/util/string.go:ValidateTypeName`
+- an empty value is accepted and resets the stored display name
+- uniqueness is not enforced; multiple users may share the same value
+- `user_name` is immutable and cannot be updated through this operation
 - reject writes while `profile_update_block` is active
-- return current aggregate on no-op rename
+- return the current aggregate on no-op updates
 
 ### Settings update