Limit concurrent image processing tasks to 4 via a tokio::Semaphore on
AppState. Acquired before spawn_blocking in both upload_image() and
save_deferred_image() to prevent CPU/memory exhaustion from concurrent
image uploads.
CPU-intensive image operations (decode, Lanczos3 resize, JPEG encode)
were running directly on the async worker thread, blocking all other
requests for 100-500ms per upload. Wrap in spawn_blocking in both
upload_image() and save_deferred_image().
Add image infrastructure, API, and detail page integration:
- EntityImage domain model with ImageRepository trait
- SQLite storage for images and thumbnails as BLOBs
- Image processing with data URL decoding and thumbnail generation
- REST API: upload, get, delete, thumbnail at /{entity_type}/{id}/image
- resolve_image_url helper for image fallback chains
- image-upload web component for direct upload on detail pages
- Image display with vignette overlay and lightbox viewer
- Template macros: image_thumbnail, readonly_image, lightbox_script
- All 7 detail pages updated with image thumbnails
- Brew images fall back to roast; cup images fall back to cafe then roast
- Delete handler macro extended with optional image cleanup
