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/images/variant_cache.ex

@@ -15,6 +15,8 @@ defmodule SimpleshopTheme.Images.VariantCache do
   alias SimpleshopTheme.Repo
   alias SimpleshopTheme.Media.Image, as: ImageSchema
   alias SimpleshopTheme.Images.{Optimizer, OptimizeWorker}
+  alias SimpleshopTheme.Products
+  alias SimpleshopTheme.Sync.ImageDownloadWorker
   import Ecto.Query
 
   @mockup_dir "priv/static/mockups"
@@ -35,6 +37,7 @@ defmodule SimpleshopTheme.Images.VariantCache do
 
     ensure_database_image_variants()
     ensure_mockup_variants()
+    ensure_product_image_downloads()
   end
 
   defp ensure_database_image_variants do
@@ -100,4 +103,15 @@ defmodule SimpleshopTheme.Images.VariantCache do
     dir = Path.dirname(source_path)
     File.exists?(Path.join(dir, "#{basename}-800.webp"))
   end
+
+  defp ensure_product_image_downloads do
+    pending = Products.list_pending_downloads(limit: 500)
+
+    if pending == [] do
+      Logger.info("[VariantCache] All product images downloaded")
+    else
+      Logger.info("[VariantCache] Enqueueing #{length(pending)} product images for download")
+      Enum.each(pending, fn image -> ImageDownloadWorker.enqueue(image.id) end)
+    end
+  end
 end