Add checkout sessions and webhook event type constants

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Id/Concerns/ManagesUserBilling.php

@@ -2,6 +2,7 @@
 
 namespace Anikeen\Id\Concerns;
 
+use Anikeen\Id\Resources\AppCheckoutSessions;
 use Anikeen\Id\Resources\AppOrders;
 use Anikeen\Id\Result;
 use Throwable;
@@ -19,6 +20,15 @@ trait ManagesUserBilling
         return new AppOrders();
     }
 
+    /**
+     * Checkout sessions of the app for any user, e.g. to look one up from a checkout.* webhook
+     * (create() then needs a user_id).
+     */
+    public function appCheckoutSessions(): AppCheckoutSessions
+    {
+        return new AppCheckoutSessions();
+    }
+
     /**
      * Create an order for a user (VAT from the billing address). With `checkout`, it is paid right
      * away: with the given `transaction` (a payment captured elsewhere) or by charging the user's