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>

priv/repo/migrations/20260131232618_add_image_id_to_product_images.exs

@@ -0,0 +1,11 @@
+defmodule SimpleshopTheme.Repo.Migrations.AddImageIdToProductImages do
+  use Ecto.Migration
+
+  def change do
+    alter table(:product_images) do
+      add :image_id, references(:images, type: :binary_id, on_delete: :nilify_all)
+    end
+
+    create index(:product_images, [:image_id])
+  end
+end