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>

PROGRESS.md

@@ -43,6 +43,11 @@
 - Disk cache for variants (regenerable from DB)
 - `mix optimize_images` task for mockups
 - On-demand JPEG fallback generation
+- Product image download pipeline (downloads Printify CDN images, processes through Media pipeline)
+  - ImageDownloadWorker downloads and links images to ProductImage
+  - PreviewData uses local images for responsive `<picture>` elements
+  - Startup recovery re-enqueues pending downloads
+  - `mix simpleshop.download_images` backfill task
 
 See: [docs/plans/image-optimization.md](docs/plans/image-optimization.md) for implementation details
 
@@ -64,6 +69,12 @@ See: [docs/plans/image-optimization.md](docs/plans/image-optimization.md) for im
 - [ ] Add variant selector component (~2hr)
 
 #### Recently Completed
+- [x] Product image download pipeline
+  - Downloads Printify CDN images via ImageDownloadWorker
+  - Processes through Media pipeline (WebP conversion, AVIF/WebP variants)
+  - PreviewData uses local images for responsive `<picture>` elements
+  - sync_product_images preserves image_id when URL unchanged
+  - Startup recovery and `mix simpleshop.download_images` backfill
 - [x] Wire shop LiveViews to Products context
   - PreviewData now uses real products when available
   - Fixed Printify image sync (position was string, not integer)