Verify Anikeen ID webhooks

Webhook::constructEvent() checks the Anikeen-Signature header (HMAC-SHA256
of "t.body" with the endpoint's secret, 5-minute tolerance) and returns
the event, like Stripe's helper.

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

src/Id/Exceptions/SignatureVerificationException.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace Anikeen\Id\Exceptions;
+
+use Exception;
+
+/**
+ * The webhook request was not signed by Anikeen ID with your endpoint's secret, or is too old.
+ */
+class SignatureVerificationException extends Exception
+{
+    //
+}