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/inttest/robot_test.go

@@ -138,36 +138,29 @@ func TestRobotPlaysAutoMatchToEnd(t *testing.T) {
 	}
 }
 
-// TestMatchmakerSubstitutesRobotEndToEnd checks a waiting human is paired with a
-// real robot account after the wait window, discoverable through Poll.
+// TestMatchmakerSubstitutesRobotEndToEnd checks the reaper fills an open game's empty
+// seat with a real robot account once its wait window has elapsed.
 func TestMatchmakerSubstitutesRobotEndToEnd(t *testing.T) {
 	ctx := context.Background()
+	clearOpenGames(t)
 	robots := newRobotService(t, newGameService())
 	if err := robots.EnsurePool(ctx); err != nil {
 		t.Fatalf("ensure pool: %v", err)
 	}
-	mm := newMatchmaker(t, robots, 10*time.Second)
+	// Zero wait and jitter so the opened game is immediately due for a robot.
+	mm := newMatchmaker(t, robots, 0, 0)
 	human := provisionAccount(t)
 
-	before := time.Now()
 	r, err := mm.Enqueue(ctx, human, engine.VariantEnglish, true)
 	if err != nil {
 		t.Fatalf("enqueue: %v", err)
 	}
 	if r.Matched {
-		t.Fatal("first enqueue must wait")
+		t.Fatal("first enqueue must open a game awaiting an opponent")
 	}
 
-	mm.Reap(ctx, before.Add(11*time.Second))
-	got, err := mm.Poll(ctx, human)
-	if err != nil {
-		t.Fatalf("poll: %v", err)
-	}
-	if !got.Matched {
-		t.Fatal("expected a substituted game after the wait window")
-	}
-
-	seats, _, status, err := newGameService().Participants(ctx, got.Game.ID)
+	mm.Reap(ctx, time.Now().Add(time.Second)) // past the (zero) wait window
+	seats, _, status, err := newGameService().Participants(ctx, r.Game.ID)
 	if err != nil {
 		t.Fatalf("participants: %v", err)
 	}