New POST /webhooks/printful route with VerifyPrintfulWebhook plug
(shared secret token via header or query param). Handles package_shipped,
order_failed, order_canceled, product_updated, product_synced, and
product_deleted events. Webhook registration via Printful v2 API with
token appended to URL. 19 new tests (819 total).
Also marks task #28 as done — Printful sync products already include
preview mockup images handled by the existing ImageDownloadWorker
pipeline.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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