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/lib/codec.test.ts

@@ -4,6 +4,7 @@ import * as fb from '../gen/fbs/scrabblefb';
 import { BLANK_INDEX, setAlphabet } from './alphabet';
 import {
   decodeDraftView,
+  decodeEvent,
   decodeFriendList,
   decodeGameList,
   decodeInvitation,
@@ -264,6 +265,19 @@ describe('codec', () => {
     expect(inv.invitees[0]).toEqual({ accountId: 'inv-1', displayName: 'Friend', seat: 1, response: 'pending' });
     expect(inv.variant).toBe('scrabble_en');
   });
+
+  it('decodes an opponent_joined event (reusing the match_found payload layout)', () => {
+    const b = new Builder(64);
+    const gid = b.createString('g-open');
+    fb.MatchFoundEvent.startMatchFoundEvent(b);
+    fb.MatchFoundEvent.addGameId(b, gid);
+    b.finish(fb.MatchFoundEvent.endMatchFoundEvent(b));
+    expect(decodeEvent('opponent_joined', b.asUint8Array())).toEqual({
+      kind: 'opponent_joined',
+      gameId: 'g-open',
+      state: undefined,
+    });
+  });
 });
 
 // The live play loop exchanges alphabet indices, mapped through the per-variant