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/providers/printify.ex

@@ -114,6 +114,66 @@ defmodule SimpleshopTheme.Providers.Printify do
     end
   end
 
+  # =============================================================================
+  # Webhook Registration
+  # =============================================================================
+
+  @webhook_events ["product:updated", "product:deleted", "product:publish:started"]
+
+  @doc """
+  Registers webhooks for product events with Printify.
+
+  Returns {:ok, results} or {:error, reason}.
+  """
+  def register_webhooks(%ProviderConnection{config: config} = conn, webhook_url) do
+    shop_id = config["shop_id"]
+    secret = config["webhook_secret"]
+
+    cond do
+      is_nil(shop_id) ->
+        {:error, :no_shop_id}
+
+      is_nil(secret) or secret == "" ->
+        {:error, :no_webhook_secret}
+
+      true ->
+        with api_key when is_binary(api_key) <- ProviderConnection.get_api_key(conn),
+             :ok <- set_api_key(api_key) do
+          results =
+            Enum.map(@webhook_events, fn event ->
+              case Client.create_webhook(shop_id, webhook_url, event, secret) do
+                {:ok, response} -> {:ok, event, response}
+                {:error, reason} -> {:error, event, reason}
+              end
+            end)
+
+          {:ok, results}
+        else
+          nil -> {:error, :no_api_key}
+        end
+    end
+  end
+
+  @doc """
+  Lists registered webhooks for the shop.
+  """
+  def list_webhooks(%ProviderConnection{config: config} = conn) do
+    shop_id = config["shop_id"]
+
+    if is_nil(shop_id) do
+      {:error, :no_shop_id}
+    else
+      with api_key when is_binary(api_key) <- ProviderConnection.get_api_key(conn),
+           :ok <- set_api_key(api_key),
+           {:ok, webhooks} <- Client.list_webhooks(shop_id) do
+        {:ok, webhooks}
+      else
+        nil -> {:error, :no_api_key}
+        {:error, _} = error -> error
+      end
+    end
+  end
+
   # =============================================================================
   # Data Normalization
   # =============================================================================