Add isInvitation field on BilletBuyer to distinguish invitations from free billets

- isInvitation (nullable bool) on BilletBuyer instead of checking totalHT == 0
- Set isInvitation=true when creating invitation in controller
- Email subject/content based on order.isInvitation
- Invoice shows organizer as buyer when order.isInvitation
- Invitations list filtered by isInvitation=true

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

migrations/Version20260321270000.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260321270000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add is_invitation to billet_buyer';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE billet_buyer ADD COLUMN IF NOT EXISTS is_invitation BOOLEAN DEFAULT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE billet_buyer DROP COLUMN IF EXISTS is_invitation');
+    }
+}