fix(multiplayer): fence client queue cancellation states

server/domain/queue.go

@@ -131,6 +131,9 @@ func (q *Queue) Cancel(playerID, ticketID, idempotencyKey string, expectedRevisi
 	if ticket.Revision != expectedRevision {
 		return QueueTicket{}, ErrStaleRevision
 	}
+	if ticket.State != Queued && ticket.State != Proposed {
+		return QueueTicket{}, fmt.Errorf("%w: cancel in %s", ErrConflict, ticket.State)
+	}
 	if idempotencyKey == "" {
 		return QueueTicket{}, fmt.Errorf("%w: empty cancel key", ErrConflict)
 	}