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>

migrations/Version20260810130000.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260810130000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add finished_at column to session table';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE session ADD finished_at DATETIME DEFAULT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE session DROP finished_at');
+    }
+}