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/simpleshop_theme/theme/preview_data.ex

@@ -185,7 +185,11 @@ defmodule SimpleshopTheme.Theme.PreviewData do
   end
 
   defp get_real_products do
-    Products.list_products(visible: true, status: "active", preload: [:images, :variants])
+    Products.list_products(
+      visible: true,
+      status: "active",
+      preload: [images: :image, variants: []]
+    )
     |> Enum.map(&product_to_map/1)
   end
 
@@ -228,16 +232,22 @@ defmodule SimpleshopTheme.Theme.PreviewData do
     in_stock = Enum.any?(available_variants)
     on_sale = Enum.any?(product.variants, &SimpleshopTheme.Products.ProductVariant.on_sale?/1)
 
+    # Use local image if available, fall back to CDN URL
+    {image_url, image_id, source_width} = image_attrs(first_image)
+    {hover_image_url, hover_image_id, hover_source_width} = image_attrs(second_image)
+
     %{
       id: product.slug,
       name: product.title,
       description: product.description,
       price: if(cheapest_variant, do: cheapest_variant.price, else: 0),
       compare_at_price: if(cheapest_variant, do: cheapest_variant.compare_at_price, else: nil),
-      image_url: if(first_image, do: first_image.src, else: nil),
-      hover_image_url: if(second_image, do: second_image.src, else: nil),
-      source_width: nil,
-      hover_source_width: nil,
+      image_url: image_url,
+      image_id: image_id,
+      hover_image_url: hover_image_url,
+      hover_image_id: hover_image_id,
+      source_width: source_width,
+      hover_source_width: hover_source_width,
       category: product.category,
       slug: product.slug,
       in_stock: in_stock,
@@ -246,6 +256,20 @@ defmodule SimpleshopTheme.Theme.PreviewData do
     }
   end
 
+  # Extract image attributes, preferring local Media.Image when available
+  defp image_attrs(nil), do: {nil, nil, nil}
+
+  defp image_attrs(%{image_id: image_id, image: %{source_width: source_width}})
+       when not is_nil(image_id) do
+    # Local image available - use image_id for responsive <picture> element
+    {nil, image_id, source_width}
+  end
+
+  defp image_attrs(%{src: src}) do
+    # Fall back to CDN URL
+    {src, nil, nil}
+  end
+
   # Default source width for mockup variants (max generated size)
   @mockup_source_width 1200