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>

tests/Entity/OrganizerInvitationTest.php

@@ -76,6 +76,21 @@ class OrganizerInvitationTest extends TestCase
         self::assertSame($inv, $result);
     }
 
+    public function testSetAndGetBillingAmount(): void
+    {
+        $inv = new OrganizerInvitation();
+
+        self::assertNull($inv->getBillingAmount());
+
+        $result = $inv->setBillingAmount(1000);
+
+        self::assertSame(1000, $inv->getBillingAmount());
+        self::assertSame($inv, $result);
+
+        $inv->setBillingAmount(0);
+        self::assertSame(0, $inv->getBillingAmount());
+    }
+
     public function testSetAndGetStatus(): void
     {
         $inv = new OrganizerInvitation();

tests/Entity/UserTest.php

@@ -187,6 +187,30 @@ class UserTest extends TestCase
         self::assertTrue($user->isStripePayoutsEnabled());
     }
 
+    public function testBillingFields(): void
+    {
+        $user = new User();
+
+        self::assertNull($user->isBilling());
+        self::assertNull($user->getBillingAmount());
+        self::assertNull($user->getBillingState());
+        self::assertNull($user->getBillingStripeSubscriptionId());
+
+        $result = $user->setIsBilling(true)
+            ->setBillingAmount(2990)
+            ->setBillingState('good')
+            ->setBillingStripeSubscriptionId('sub_123456');
+
+        self::assertSame($user, $result);
+        self::assertTrue($user->isBilling());
+        self::assertSame(2990, $user->getBillingAmount());
+        self::assertSame('good', $user->getBillingState());
+        self::assertSame('sub_123456', $user->getBillingStripeSubscriptionId());
+
+        $user->setBillingState('suspendu');
+        self::assertSame('suspendu', $user->getBillingState());
+    }
+
     public function testEmailVerificationFields(): void
     {
         $user = new User();