[DRAFT] update metadata/covers on kobo sync

[tsheinen]

I have initial support for updating metadata and covers on kobo sync. This is a draft and shouldn't be merged yet but I wanted to get a second pair of eyes on it.

I have a few specific things I'd like thoughts on.

• Current design for handling hashed covers is bad/wrong when two books share the same cover. We could store thumbnails in sqlite or maybe store path to the cover on disk?
• What does ChangedEntitlement do? It didn't seem to do anything in my testing.
• How should we handle removing a book from KoboSyncedBooks when it changes? It's easy to just remove it when modified through the UI but ideally we should support filesystem changes too. Calibre's last_modified metadata column could be used but seems generally likely to be spuriously true. We could store last sync in KoboSyncedBooks and compare against last modified time on disk? Could also add an API route to resync a calibre id. Thoughts?

Thanks!