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.

ui/src/screens/Lobby.svelte

@@ -52,6 +52,8 @@
   const groups = $derived(groupGames(games, myId));
 
   function opponents(g: GameView): string {
+    // An auto-match game still waiting for an opponent shows the "searching" placeholder.
+    if (g.status === 'open') return t('game.searchingForOpponent');
     return g.seats
       .filter((s) => s.accountId !== myId)
       .map((s) => s.displayName)