feat: add product image download pipeline for PageSpeed 100%

Downloads Printify CDN images via ImageDownloadWorker, processes
through Media pipeline (WebP conversion, AVIF/WebP variant generation),
and links to ProductImage via new image_id FK.

- Add image_id to product_images table
- ImageDownloadWorker downloads and processes external images
- sync_product_images preserves image_id when URL unchanged
- PreviewData uses local images for responsive <picture> elements
- VariantCache enqueues pending downloads on startup
- mix simpleshop.download_images backfill task

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

lib/mix/tasks/simpleshop/download_images.ex

@@ -0,0 +1,41 @@
+defmodule Mix.Tasks.Simpleshop.DownloadImages do
+  @moduledoc """
+  Enqueues download jobs for product images that haven't been processed yet.
+
+  ## Usage
+
+      mix simpleshop.download_images
+
+  This task finds all product images with a `src` URL but no linked `image_id`
+  and enqueues them for download via the ImageDownloadWorker.
+
+  Use this to backfill existing products after enabling the image download pipeline.
+  """
+
+  use Mix.Task
+
+  @shortdoc "Enqueue product image downloads"
+
+  @impl Mix.Task
+  def run(_args) do
+    Mix.Task.run("app.start")
+
+    alias SimpleshopTheme.Products
+    alias SimpleshopTheme.Sync.ImageDownloadWorker
+
+    pending = Products.list_pending_downloads(limit: 10_000)
+    count = length(pending)
+
+    if count == 0 do
+      Mix.shell().info("No pending product images to download.")
+    else
+      Mix.shell().info("Enqueueing #{count} product images for download...")
+
+      Enum.each(pending, fn image ->
+        ImageDownloadWorker.enqueue(image.id)
+      end)
+
+      Mix.shell().info("Done. Images will be processed by Oban workers.")
+    end
+  end
+end