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/components/page_templates/pdp.html.heex

@@ -19,16 +19,24 @@
 
   <main id="main-content" class="max-w-7xl mx-auto px-4 sm:px-6 lg:px-8 py-8">
     <.breadcrumb
-      items={[
-        %{label: "Home", page: "home", href: "/"},
-        %{
-          label: @product.category,
-          page: "collection",
-          href:
-            "/collections/#{@product.category |> String.downcase() |> String.replace(" ", "-")}"
-        },
-        %{label: @product.name, current: true}
-      ]}
+      items={
+        [
+          %{label: "Home", page: "home", href: "/"}
+        ] ++
+          if @product.category do
+            [
+              %{
+                label: @product.category,
+                page: "collection",
+                href:
+                  "/collections/#{@product.category |> String.downcase() |> String.replace(" ", "-")}"
+              }
+            ]
+          else
+            []
+          end ++
+          [%{label: @product.name, current: true}]
+      }
       mode={@mode}
     />