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/model.ts

@@ -5,8 +5,9 @@
 
 export type Variant = 'scrabble_en' | 'scrabble_ru' | 'erudit_ru';
 
-/** Backend game status strings. */
-export type GameStatus = 'active' | 'finished' | string;
+/** Backend game status strings. 'open' is an auto-match game the player has entered
+ *  but which is still waiting for an opponent (the opponent seat has no account). */
+export type GameStatus = 'active' | 'finished' | 'open' | string;
 
 /** Decoded move action kinds (history-independent, see ARCHITECTURE §9.1). */
 export type MoveAction = 'play' | 'pass' | 'exchange' | 'resign' | 'timeout' | string;
@@ -237,8 +238,8 @@ export interface GameList {
 /**
  * A live event delivered over the Subscribe stream. The game events carry the move as a
  * delta — move plus the post-move summary (and the bag size) — the client applies to its
- * cached game without a refetch; match_found / game_started carry the recipient's initial
- * StateView; notify carries the changed lobby payload. The enriched fields are optional
+ * cached game without a refetch; match_found / game_started / opponent_joined carry the
+ * recipient's StateView; notify carries the changed lobby payload. The enriched fields are optional
  * so a client falls back to a refetch when a payload is absent (a gap, or an older peer).
  */
 export type PushEvent =
@@ -248,5 +249,6 @@ export type PushEvent =
   | { kind: 'chat_message'; message: ChatMessage }
   | { kind: 'nudge'; gameId: string; fromUserId: string }
   | { kind: 'match_found'; gameId: string; state?: StateView }
+  | { kind: 'opponent_joined'; gameId: string; state?: StateView }
   | { kind: 'notify'; sub: string; account?: AccountRef; invitation?: Invitation; state?: StateView }
   | { kind: 'heartbeat' };