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.

backend/internal/notify/events.go

@@ -122,6 +122,22 @@ func MatchFound(userID, gameID uuid.UUID, state PlayerState) Intent {
 	return Intent{UserID: userID, Kind: KindMatchFound, Payload: b.FinishedBytes(), EventID: eventID()}
 }
 
+// OpponentJoined tells userID — the starter of an auto-match game still shown as
+// "searching for an opponent" — that an opponent (a human or a substituted robot) has
+// taken the empty seat. state is the starter's refreshed view (now seating both
+// players), so the client fills the opponent card and re-enables resign and chat in
+// place without navigating. It reuses the match_found payload layout (game id + state).
+func OpponentJoined(userID, gameID uuid.UUID, state PlayerState) Intent {
+	b := flatbuffers.NewBuilder(512)
+	gid := b.CreateString(gameID.String())
+	stateOff := buildStateView(b, state)
+	fb.MatchFoundEventStart(b)
+	fb.MatchFoundEventAddGameId(b, gid)
+	fb.MatchFoundEventAddState(b, stateOff)
+	b.Finish(fb.MatchFoundEventEnd(b))
+	return Intent{UserID: userID, Kind: KindOpponentJoined, Payload: b.FinishedBytes(), EventID: eventID()}
+}
+
 // Notification is a lightweight "re-poll" signal to userID that something in their lobby
 // changed. kind is a sub-discriminator (NotifyFriendRequest, NotifyFriendAdded,
 // NotifyFriendDeclined, NotifyInvitation, NotifyGameStarted). It carries no payload; prefer the