Keep pregame lobby chat open for an hour after the game ends

The chat used to disappear the moment a session left CREATED status.
Sessions now record a finishedAt timestamp when they're won or lost,
and the lobby (with chat) stays reachable via /game/{session} for an
hour afterward instead of immediately redirecting to the win/lose
feedback page. The lobby template shows a distinct "game finished"
header with a link to that feedback page during this window.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/Game/Controller/GameAdminSessionController.php

@@ -35,6 +35,7 @@ final class GameAdminSessionController extends AbstractController
         }
 
         $session->setStatus(SessionStatus::LOST);
+        $session->setFinishedAt(new \DateTime());
         $em->flush();
 
         $this->addFlash('success', sprintf('Session #%d closed.', $session->getId()));