Add checkout sessions and webhook event type constants

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

src/Id/Resources/CheckoutSessions.php

@@ -0,0 +1,58 @@
+<?php
+
+namespace Anikeen\Id\Resources;
+
+use Anikeen\Id\Concerns\HasBillable;
+use Anikeen\Id\Contracts\Billable;
+use Anikeen\Id\Exceptions\ResourceException;
+use Illuminate\Database\Eloquent\Model;
+use Throwable;
+
+/**
+ * Checkout sessions of the signed-in user, with the user's access token (any app): several
+ * subscriptions the user confirms together on Anikeen ID. Nothing is requested until a method is
+ * called.
+ */
+class CheckoutSessions
+{
+    use HasBillable;
+
+    public function __construct(Billable|Model $billable)
+    {
+        $this->billable = $billable;
+    }
+
+    /**
+     * Open a checkout session with new subscriptions and/or existing drafts of the app
+     * (`['id' => …]`); see AppCheckoutSessions::create() for the attributes (no user_id here,
+     * vat_rate only for trusted apps).
+     *
+     * @throws Throwable
+     */
+    public function create(array $attributes): CheckoutSession
+    {
+        return (new CheckoutSession(fn () => $this->billable->anikeenId()
+            ->request('POST', 'v1/checkout-sessions', $attributes)))
+            ->setBillable($this->billable);
+    }
+
+    /**
+     * A session the app created for the user; null when it doesn't exist.
+     *
+     * @throws Throwable
+     */
+    public function find(string $id): ?CheckoutSession
+    {
+        try {
+            return (new CheckoutSession(fn () => $this->billable->anikeenId()
+                ->request('GET', sprintf('v1/checkout-sessions/%s', $id))))
+                ->setBillable($this->billable);
+        } catch (ResourceException $e) {
+            if ($e->getCode() === 404) {
+                return null;
+            }
+
+            throw $e;
+        }
+    }
+}