feat(core): webhook registry for addon events

[mckenziearts]

What

Adds a WebhookRegistry so packages and addons can register their own webhook events and payload serializers at runtime, instead of relying on config()->set writes.

use Shopper\Core\Webhooks\Facades\Webhooks;

Webhooks::register(SubscriptionRenewed::class, 'subscription.renewed', SubscriptionPayloadSerializer::class);

The serializer is optional (class implementing WebhookPayloadSerializer or a closure). Without one, the event falls back to the bound default serializer. Registered events automatically appear in the admin subscription form and are dispatched through the existing delivery pipeline (signature, retries, auto-disable).

How

• The registry seeds the core events from shopper.webhooks.events lazily and treats them as defaults: code can override their public name and attach a serializer to them without conflict.
• Conflicts between two code registrations fail fast with a LogicException; a duplicate public name is logged and ignored so an addon collision never prevents the store from booting.
• Listeners are attached once the application has booted, so webhook payloads always snapshot the state after the application's own listeners ran. Events registered after boot are attached immediately.
• The listener attach is idempotent against the dispatcher, so recycled registry instances (Octane) can never stack duplicate deliveries.
• Malformed config entries and empty payloads are logged once instead of failing the request, and a registered serializer returning a malformed shape is normalised so the delivery is never silently lost.
• CartServiceProvider now registers cart.completed through the facade.