Add debt system: track refunds/disputes, redirect payments until cleared

- Add nullable debt field to User entity with addDebt/reduceDebt helpers
- On refund webhook: add refunded amount to organizer debt
- On dispute webhook (charge.dispute.created): add disputed amount to debt
- OrderController: if organizer has debt > 0, payment goes to main Stripe
  account instead of connected account, debt reduced on payment success
- Display debt amount on organizer dashboard with warning message
- Add dispute notification email template
- Migration for debt column on user table

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

migrations/Version20260325090632.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260325090632 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add debt field to user table for refund/dispute tracking';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE "user" ADD debt INT DEFAULT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE "user" DROP debt');
+    }
+}