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>

migrations/Version20260810120000.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260810120000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add lobby_message table for pre-game lobby chat';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('CREATE TABLE lobby_message (id INT AUTO_INCREMENT NOT NULL, session_id INT NOT NULL, player_id INT NOT NULL, content VARCHAR(500) NOT NULL, created_at DATETIME NOT NULL, INDEX IDX_LOBBY_MESSAGE_SESSION (session_id), INDEX IDX_LOBBY_MESSAGE_PLAYER (player_id), PRIMARY KEY(id)) DEFAULT CHARACTER SET utf8mb4 COLLATE `utf8mb4_unicode_ci` ENGINE = InnoDB');
+        $this->addSql('ALTER TABLE lobby_message ADD CONSTRAINT FK_LOBBY_MESSAGE_SESSION FOREIGN KEY (session_id) REFERENCES session (id)');
+        $this->addSql('ALTER TABLE lobby_message ADD CONSTRAINT FK_LOBBY_MESSAGE_PLAYER FOREIGN KEY (player_id) REFERENCES player (id)');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE lobby_message DROP FOREIGN KEY FK_LOBBY_MESSAGE_SESSION');
+        $this->addSql('ALTER TABLE lobby_message DROP FOREIGN KEY FK_LOBBY_MESSAGE_PLAYER');
+        $this->addSql('DROP TABLE lobby_message');
+    }
+}