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>

test/simpleshop_theme/webhooks/product_delete_worker_test.exs

@@ -0,0 +1,48 @@
+defmodule SimpleshopTheme.Webhooks.ProductDeleteWorkerTest do
+  use SimpleshopTheme.DataCase
+  use Oban.Testing, repo: SimpleshopTheme.Repo
+
+  alias SimpleshopTheme.Webhooks.ProductDeleteWorker
+  alias SimpleshopTheme.Products
+
+  import SimpleshopTheme.ProductsFixtures
+
+  describe "perform/1" do
+    test "deletes product when found" do
+      conn = provider_connection_fixture(%{provider_type: "printify"})
+
+      {:ok, product, :created} =
+        Products.upsert_product(conn, %{
+          provider_product_id: "test-product-123",
+          title: "Test Product",
+          provider_data: %{}
+        })
+
+      assert Products.get_product(product.id) != nil
+
+      assert :ok =
+               perform_job(ProductDeleteWorker, %{
+                 provider_product_id: "test-product-123"
+               })
+
+      assert Products.get_product(product.id) == nil
+    end
+
+    test "returns ok when product not found" do
+      _conn = provider_connection_fixture(%{provider_type: "printify"})
+
+      assert :ok =
+               perform_job(ProductDeleteWorker, %{
+                 provider_product_id: "nonexistent-product"
+               })
+    end
+
+    test "cancels when no provider connection" do
+      # No connection created
+      assert {:cancel, :no_connection} =
+               perform_job(ProductDeleteWorker, %{
+                 provider_product_id: "test-product-123"
+               })
+    end
+  end
+end