Add offer and commission rate to organizer invitation

- offer (free/basic/custom) and commissionRate fields on OrganizerInvitation
- Admin form: select offer + commission rate input
- Invitation list: show offer badge + rate
- Email: gold banner with proposed offer and commission rate (hors Stripe)

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

tests/Entity/OrganizerInvitationTest.php

@@ -17,6 +17,8 @@ class OrganizerInvitationTest extends TestCase
         self::assertNull($inv->getLastName());
         self::assertNull($inv->getEmail());
         self::assertNull($inv->getMessage());
+        self::assertNull($inv->getOffer());
+        self::assertNull($inv->getCommissionRate());
         self::assertSame(OrganizerInvitation::STATUS_SENT, $inv->getStatus());
         self::assertSame(64, \strlen($inv->getToken()));
         self::assertNull($inv->getRespondedAt());
@@ -56,6 +58,24 @@ class OrganizerInvitationTest extends TestCase
         self::assertNull($inv->getMessage());
     }
 
+    public function testSetAndGetOffer(): void
+    {
+        $inv = new OrganizerInvitation();
+        $result = $inv->setOffer('basic');
+
+        self::assertSame('basic', $inv->getOffer());
+        self::assertSame($inv, $result);
+    }
+
+    public function testSetAndGetCommissionRate(): void
+    {
+        $inv = new OrganizerInvitation();
+        $result = $inv->setCommissionRate(5.5);
+
+        self::assertSame(5.5, $inv->getCommissionRate());
+        self::assertSame($inv, $result);
+    }
+
     public function testSetAndGetStatus(): void
     {
         $inv = new OrganizerInvitation();