Add organizer invitation system: invite, accept, refuse

- OrganizerInvitation entity: companyName, firstName, lastName, email,
  message, status (sent/opened/accepted/refused), unique token (64 hex chars)
- Admin route /admin/organisateurs/inviter: form + invitation list with status
- Button "Inviter un organisateur" on admin organizers page
- Email with accept/refuse links using unique token
- Public route /invitation/{token}/{action}: accept or refuse without auth
- Response page: confirmation message for accept/refuse
- Migration, PHPStan config, 7 entity tests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

migrations/Version20260322100000.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260322100000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Create organizer_invitation table';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('CREATE TABLE organizer_invitation (id INT GENERATED BY DEFAULT AS IDENTITY NOT NULL, company_name VARCHAR(255) NOT NULL, first_name VARCHAR(255) NOT NULL, last_name VARCHAR(255) NOT NULL, email VARCHAR(255) NOT NULL, message TEXT DEFAULT NULL, status VARCHAR(20) NOT NULL, token VARCHAR(64) NOT NULL, created_at TIMESTAMP(0) WITHOUT TIME ZONE NOT NULL, responded_at TIMESTAMP(0) WITHOUT TIME ZONE DEFAULT NULL, PRIMARY KEY(id))');
+        $this->addSql('CREATE UNIQUE INDEX UNIQ_ORG_INV_TOKEN ON organizer_invitation (token)');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('DROP TABLE organizer_invitation');
+    }
+}