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/webhooks/product_delete_worker.ex

@@ -0,0 +1,41 @@
+defmodule SimpleshopTheme.Webhooks.ProductDeleteWorker do
+  @moduledoc """
+  Oban worker for deleting products removed from POD providers.
+  """
+
+  use Oban.Worker, queue: :sync, max_attempts: 3
+
+  alias SimpleshopTheme.Products
+
+  require Logger
+
+  @impl Oban.Worker
+  def perform(%Oban.Job{args: %{"provider_product_id" => provider_product_id}}) do
+    case Products.get_provider_connection_by_type("printify") do
+      nil ->
+        Logger.warning("No Printify connection found for product deletion")
+        {:cancel, :no_connection}
+
+      conn ->
+        case Products.get_product_by_provider(conn.id, provider_product_id) do
+          nil ->
+            Logger.info("Product #{provider_product_id} already deleted or not found")
+            :ok
+
+          product ->
+            Logger.info("Deleting product #{product.id} (provider: #{provider_product_id})")
+
+            case Products.delete_product(product) do
+              {:ok, _} -> :ok
+              {:error, reason} -> {:error, reason}
+            end
+        end
+    end
+  end
+
+  def enqueue(provider_product_id) do
+    %{provider_product_id: to_string(provider_product_id)}
+    |> new()
+    |> Oban.insert()
+  end
+end