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/game/ChatScreen.svelte

@@ -17,7 +17,11 @@
   let tick = $state(0);
 
   const myId = $derived(app.session?.userId ?? '');
-  const isMyTurn = $derived(!!view && view.game.status === 'active' && view.game.toMove === view.seat);
+  const isMyTurn = $derived(
+    !!view && (view.game.status === 'active' || view.game.status === 'open') && view.game.toMove === view.seat,
+  );
+  // While the auto-match game still has no opponent, chat and nudge are both disabled.
+  const waiting = $derived(!!view && view.game.status === 'open');
   const nudgeCooldownSecs = 3600;
   // The nudge is one-per-hour-per-game and clears once the player chats (engagement); the
   // backend stays authoritative, so a move-based reset is left to it.
@@ -87,4 +91,4 @@
   }
 </script>
 
-<Chat {messages} {myId} {busy} myTurn={isMyTurn} {nudgeOnCooldown} onsend={sendChat} onnudge={nudge} />
+<Chat {messages} {myId} {busy} myTurn={isMyTurn} {waiting} {nudgeOnCooldown} onsend={sendChat} onnudge={nudge} />