feat: add Printify webhook endpoint for real-time product updates

- Add /webhooks/printify endpoint with HMAC-SHA256 signature verification
- Add Webhooks context to handle product:updated, product:deleted events
- Add ProductDeleteWorker for async product deletion
- Add webhook API methods to Printify client (create, list, delete)
- Add register_webhooks/2 to Printify provider
- Add mix register_webhooks task for one-time webhook registration
- Cache raw request body in endpoint for signature verification

Usage:
1. Generate webhook secret: openssl rand -hex 20
2. Add to provider connection config as "webhook_secret"
3. Register with Printify: mix register_webhooks https://yourshop.com/webhooks/printify

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/simpleshop_theme/clients/printify.ex

@@ -183,6 +183,41 @@ defmodule SimpleshopTheme.Clients.Printify do
     get("/shops/#{shop_id}/orders/#{order_id}.json")
   end
 
+  # =============================================================================
+  # Webhooks
+  # =============================================================================
+
+  @doc """
+  Register a webhook with Printify.
+
+  ## Event types
+  - "product:publish:started"
+  - "product:updated"
+  - "product:deleted"
+  - "shop:disconnected"
+  """
+  def create_webhook(shop_id, url, topic, secret) do
+    post("/shops/#{shop_id}/webhooks.json", %{
+      topic: topic,
+      url: url,
+      secret: secret
+    })
+  end
+
+  @doc """
+  List registered webhooks for a shop.
+  """
+  def list_webhooks(shop_id) do
+    get("/shops/#{shop_id}/webhooks.json")
+  end
+
+  @doc """
+  Delete a webhook.
+  """
+  def delete_webhook(shop_id, webhook_id) do
+    delete("/shops/#{shop_id}/webhooks/#{webhook_id}.json")
+  end
+
   @doc """
   Download a file from a URL to a local path.
   """