- Centralize cover image processing into a new `imageService.processCoverImage()` helper that resizes and crops images to 1000×1000 JPEG.
- Replace ad-hoc embedded-art handling in admin routes (`writeEmbeddedCover`) and publish route (`uploadCover`) with unified `saveCover` function supporting both uploaded files and embedded picture buffers.
- Ensure all cover uploads (songs, playlists, generated songs) go through the new async processing pipeline with proper temp-file cleanup and error handling.
- Update routes to use `async` handlers where cover processing is involved.
