feat(lobby): enter the game immediately and wait for the opponent inside it

Quick auto-match no longer waits on a separate screen: Enqueue opens a real game seating the caller with an empty opponent seat (new game status 'open') and the player enters it at once. A second human searching the same variant+rule joins that open game; otherwise a background reaper seats a robot after a 90s + random 0-90s wait, pushing a new in-app opponent_joined event that fills the opponent card and re-enables resign and chat in place.

Matchmaking state is now the open games in the database (the in-memory pool, lobby.poll and lobby.cancel are gone), serialised by a per-bucket advisory lock. While a game is open the starter may move on their turn, but resign, chat and nudge are refused; the lobby and opponent card show "searching for opponent".

Schema edited in the baseline (no prod data): 'open' status, nullable game_players.account_id for the empty seat, and a games.open_deadline_at stamp; jet code regenerated.

backend/internal/config/config.go

@@ -100,6 +100,9 @@ func Load() (Config, error) {
 	if lb.RobotWait, err = envDuration("BACKEND_LOBBY_ROBOT_WAIT", lb.RobotWait); err != nil {
 		return Config{}, err
 	}
+	if lb.RobotWaitJitter, err = envDuration("BACKEND_LOBBY_ROBOT_WAIT_JITTER", lb.RobotWaitJitter); err != nil {
+		return Config{}, err
+	}
 	if lb.ReaperInterval, err = envDuration("BACKEND_LOBBY_REAPER_INTERVAL", lb.ReaperInterval); err != nil {
 		return Config{}, err
 	}