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>

tests/Entity/UserTest.php

@@ -211,6 +211,32 @@ class UserTest extends TestCase
         self::assertSame('suspendu', $user->getBillingState());
     }
 
+    public function testDebtFields(): void
+    {
+        $user = new User();
+
+        self::assertNull($user->getDebt());
+
+        $result = $user->setDebt(5000);
+        self::assertSame($user, $result);
+        self::assertSame(5000, $user->getDebt());
+
+        $user->addDebt(2000);
+        self::assertSame(7000, $user->getDebt());
+
+        $user->reduceDebt(3000);
+        self::assertSame(4000, $user->getDebt());
+
+        $user->reduceDebt(10000);
+        self::assertSame(0, $user->getDebt());
+
+        $user->setDebt(null);
+        self::assertNull($user->getDebt());
+
+        $user->addDebt(1000);
+        self::assertSame(1000, $user->getDebt());
+    }
+
     public function testEmailVerificationFields(): void
     {
         $user = new User();