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_web/live/shop_live/product_show.ex

@@ -34,13 +34,11 @@ defmodule SimpleshopThemeWeb.ShopLive.ProductShow do
       |> Enum.reject(fn p -> p.id == product.id end)
       |> Enum.take(4)
 
-    # Build gallery images (filter out nils)
+    # Build gallery images from local image_id or external URL
     gallery_images =
       [
-        product.image_url,
-        product.hover_image_url,
-        product.image_url,
-        product.hover_image_url
+        image_src(product[:image_id], product[:image_url]),
+        image_src(product[:hover_image_id], product[:hover_image_url])
       ]
       |> Enum.reject(&is_nil/1)
 
@@ -70,6 +68,14 @@ defmodule SimpleshopThemeWeb.ShopLive.ProductShow do
       List.first(products)
   end
 
+  # Build image source URL - prefer local image_id, fall back to external URL
+  defp image_src(image_id, _url) when is_binary(image_id) do
+    "/images/#{image_id}/variant/1200.webp"
+  end
+
+  defp image_src(_, url) when is_binary(url), do: url
+  defp image_src(_, _), do: nil
+
   @impl true
   def render(assigns) do
     ~H"""