Add pregame lobby with live chat

Players used to get bounced back to the dashboard when a session
wasn't full yet. Now they land on a lobby page showing who has
joined, and can chat with each other while waiting - messages are
broadcast live over the existing Mercure hub, and the session
auto-starts (and the lobby notifies everyone) once it fills up.

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

src/Game/Repository/LobbyMessageRepository.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace App\Game\Repository;
+
+use App\Game\Entity\LobbyMessage;
+use App\Game\Entity\Session;
+use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
+use Doctrine\Persistence\ManagerRegistry;
+
+/**
+ * @extends ServiceEntityRepository<LobbyMessage>
+ */
+class LobbyMessageRepository extends ServiceEntityRepository
+{
+    public function __construct(ManagerRegistry $registry)
+    {
+        parent::__construct($registry, LobbyMessage::class);
+    }
+
+    /**
+     * @return LobbyMessage[]
+     */
+    public function findForSession(Session $session, int $limit = 100): array
+    {
+        return $this->createQueryBuilder('m')
+            ->andWhere('m.session = :session')
+            ->setParameter('session', $session)
+            ->orderBy('m.createdAt', 'ASC')
+            ->setMaxResults($limit)
+            ->getQuery()
+            ->getResult();
+    }
+}