Add billing system: subscription, webhooks, and access control

- Add billing fields to User (isBilling, billingAmount, billingState,
  billingStripeSubscriptionId) and OrganizerInvitation (billingAmount)
- Registration: organizer gets billingState="poor" (pending review)
- Admin approval: sets isBilling=true, billingAmount from form, state="good"
- Invitation: billingAmount from invitation, if 0 then isBilling=false
- ROLE_ROOT accounts: billing free (amount=0, state="good")
- Block Stripe Connect creation and all organizer features if state is
  "poor" or "suspendu"
- Hide Stripe configuration section if billing not settled
- Add billing checkout via Stripe subscription with success route
- Webhooks: checkout.session.completed activates billing,
  invoice.payment_failed and customer.subscription.deleted suspend
  account and disable online events
- Show billing alert on /mon-compte with amount and subscribe button
- Display billing info in invitation email and landing page
- Add email templates for billing activated/failed/cancelled

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

migrations/Version20260324131819.php

@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260324131819 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add billing fields to user and organizer_invitation tables';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE organizer_invitation ADD billing_amount INT DEFAULT NULL');
+        $this->addSql('ALTER TABLE "user" ADD is_billing BOOLEAN DEFAULT NULL');
+        $this->addSql('ALTER TABLE "user" ADD billing_amount INT DEFAULT NULL');
+        $this->addSql('ALTER TABLE "user" ADD billing_state VARCHAR(20) DEFAULT NULL');
+        $this->addSql('ALTER TABLE "user" ADD billing_stripe_subscription_id VARCHAR(255) DEFAULT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE organizer_invitation DROP billing_amount');
+        $this->addSql('ALTER TABLE "user" DROP is_billing');
+        $this->addSql('ALTER TABLE "user" DROP billing_amount');
+        $this->addSql('ALTER TABLE "user" DROP billing_state');
+        $this->addSql('ALTER TABLE "user" DROP billing_stripe_subscription_id');
+    }
+}