After /tool/signup, the client has a viewer_id but no Steam social row.
The first authenticated request (typically /check/game_start) carries
the Steam ticket; if the SteamId lookup misses but the UDID resolves
to a viewer, attach the Steam social now. Subsequent requests hit the
fast SteamId path. Closes the CheckController.GameStart TODO that was
blocking fresh-client boot.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
