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/mix/tasks/register_webhooks.ex

@@ -0,0 +1,69 @@
+defmodule Mix.Tasks.RegisterWebhooks do
+  @moduledoc """
+  Registers Printify webhooks for a shop.
+
+  Usage:
+      mix register_webhooks https://yourshop.com/webhooks/printify
+
+  Prerequisites:
+  - Printify provider connection must exist with shop_id configured
+  - webhook_secret must be set in the connection config
+
+  Generate a webhook secret with:
+      openssl rand -hex 20
+  """
+
+  use Mix.Task
+
+  @shortdoc "Register Printify webhooks for product updates"
+
+  @impl Mix.Task
+  def run([url]) do
+    Mix.Task.run("app.start")
+
+    alias SimpleshopTheme.Products
+    alias SimpleshopTheme.Providers.Printify
+
+    case Products.get_provider_connection_by_type("printify") do
+      nil ->
+        Mix.shell().error("No Printify connection found. Create one at /admin/providers first.")
+
+      conn ->
+        Mix.shell().info("Registering webhooks for URL: #{url}")
+
+        case Printify.register_webhooks(conn, url) do
+          {:ok, results} ->
+            Enum.each(results, fn
+              {:ok, event, response} ->
+                Mix.shell().info("✓ Registered: #{event} (ID: #{response["id"]})")
+
+              {:error, event, reason} ->
+                Mix.shell().error("✗ Failed: #{event} - #{inspect(reason)}")
+            end)
+
+            Mix.shell().info("\nDone!")
+
+          {:error, :no_webhook_secret} ->
+            Mix.shell().error("""
+            No webhook secret configured.
+
+            Generate one with: openssl rand -hex 20
+
+            Then add it to your provider connection config as "webhook_secret".
+            """)
+
+          {:error, reason} ->
+            Mix.shell().error("Failed: #{inspect(reason)}")
+        end
+    end
+  end
+
+  def run(_) do
+    Mix.shell().error("""
+    Usage: mix register_webhooks <url>
+
+    Example:
+        mix register_webhooks https://yourshop.com/webhooks/printify
+    """)
+  end
+end