Books.atom_timestamp returned Books.timestamp (date added), which is
set at import and never changes. OPDS clients use atom:updated to
decide whether a book has changed on the server, so cover swaps,
metadata edits, and any other post-import change were invisible to
sync clients; they would keep serving the stale cover and title
until a manual refresh.
Atom RFC 4287 defines updated as "the most recent instant in time
when an entry or feed was modified", and Calibre already tracks that
field as last_modified, bumping it on every metadata and cover edit.
Switching the property to return last_modified (with a fallback to
timestamp when last_modified is NULL) aligns Calibre-Web's behaviour
with the Atom contract.
This change only affects the OPDS feed's atom:updated element. Kobo
sync uses its own last_modified comparison path, so it is unaffected.
