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.ts

@@ -465,6 +465,12 @@ export function decodeEvent(kind: string, payload: Uint8Array): PushEvent | null
       const st = e.state();
       return { kind: 'match_found', gameId: s(e.gameId()), state: st ? decodeStateViewTable(st) : undefined };
     }
+    case 'opponent_joined': {
+      // opponent_joined reuses the match_found payload layout (game id + the recipient's state).
+      const e = fb.MatchFoundEvent.getRootAsMatchFoundEvent(bb);
+      const st = e.state();
+      return { kind: 'opponent_joined', gameId: s(e.gameId()), state: st ? decodeStateViewTable(st) : undefined };
+    }
     case 'notify': {
       const e = fb.NotificationEvent.getRootAsNotificationEvent(bb);
       const acc = e.account();