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/server/handlers.go

@@ -76,8 +76,6 @@ func (s *Server) registerRoutes() {
 	}
 	if s.matchmaker != nil {
 		u.POST("/lobby/enqueue", s.handleEnqueue)
-		u.POST("/lobby/cancel", s.handleCancel)
-		u.GET("/lobby/poll", s.handlePoll)
 	}
 	if s.invitations != nil {
 		u.GET("/invitations", s.handleListInvitations)
@@ -161,14 +159,14 @@ func statusForError(err error) (int, string) {
 		return http.StatusConflict, "nudge_own_turn"
 	case errors.Is(err, game.ErrFinished), errors.Is(err, social.ErrGameNotActive):
 		return http.StatusConflict, "game_finished"
+	case errors.Is(err, game.ErrNoOpponentYet):
+		return http.StatusConflict, "no_opponent_yet"
 	case errors.Is(err, game.ErrGameActive):
 		return http.StatusConflict, "game_active"
 	case errors.Is(err, account.ErrInvalidProfile):
 		return http.StatusBadRequest, "invalid_profile"
 	case errors.Is(err, account.ErrAlreadyConfirmed):
 		return http.StatusConflict, "already_confirmed"
-	case errors.Is(err, lobby.ErrAlreadyQueued):
-		return http.StatusConflict, "already_queued"
 	case errors.Is(err, lobby.ErrInvalidInvitation):
 		return http.StatusBadRequest, "invalid_invitation"
 	case errors.Is(err, lobby.ErrInvitationBlocked):