Design cloud photo storage with sync across devices

Design a photo library in the cloud that keeps every one of a user's devices in sync: upload new photos from a phone, see them on the other devices, and propagate edits and deletes. Cover uploading large files over flaky mobile networks, storing originals and thumbnails, the metadata service, how devices learn about changes (push versus polling, change logs), conflicts when two devices edit the same photo, offline use, and storage cost at scale.