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/game/eventwire.go

@@ -1,6 +1,8 @@
 package game
 
 import (
+	"github.com/google/uuid"
+
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/notify"
 )
@@ -21,9 +23,15 @@ func gameSummary(g Game, names []string) notify.GameSummary {
 		if s.Seat >= 0 && s.Seat < len(names) {
 			name = names[s.Seat]
 		}
+		// An open game's still-empty opponent seat carries no account: send an empty id
+		// (not the nil-UUID string) so the client renders it as "searching for opponent".
+		accountID := ""
+		if s.AccountID != uuid.Nil {
+			accountID = s.AccountID.String()
+		}
 		seats = append(seats, notify.SeatStanding{
 			Seat:        s.Seat,
-			AccountID:   s.AccountID.String(),
+			AccountID:   accountID,
 			DisplayName: name,
 			Score:       s.Score,
 			HintsUsed:   s.HintsUsed,