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/notify.go

@@ -24,6 +24,11 @@ const (
 	KindChatMessage   = "chat_message"
 	KindNudge         = "nudge"
 	KindMatchFound    = "match_found"
+	// KindOpponentJoined tells the starter of an auto-match game still "searching for an
+	// opponent" that the empty seat has been taken (by a human or a substituted robot),
+	// carrying the refreshed StateView so the client fills the opponent card and
+	// re-enables resign and chat in place. In-app only (never an out-of-app push).
+	KindOpponentJoined = "opponent_joined"
 	// KindNotification is a lightweight "re-poll your lobby counters" signal
 	// (incoming friend requests, invitations) that drives the lobby badge.
 	KindNotification = "notify"