Add offer and commission rate to organizer invitation

- offer (free/basic/custom) and commissionRate fields on OrganizerInvitation
- Admin form: select offer + commission rate input
- Invitation list: show offer badge + rate
- Email: gold banner with proposed offer and commission rate (hors Stripe)

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

migrations/Version20260322110000.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260322110000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add offer and commission_rate to organizer_invitation';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE organizer_invitation ADD COLUMN IF NOT EXISTS offer VARCHAR(20) DEFAULT NULL');
+        $this->addSql('ALTER TABLE organizer_invitation ADD COLUMN IF NOT EXISTS commission_rate DOUBLE PRECISION DEFAULT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE organizer_invitation DROP COLUMN IF EXISTS offer');
+        $this->addSql('ALTER TABLE organizer_invitation DROP COLUMN IF EXISTS commission_rate');
+    }
+}