fix(opds): atom:updated reflects last modification, not date added

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.

cps/db.py

@@ -449,7 +449,16 @@ class Books(Base):
 
     @property
     def atom_timestamp(self):
-        return self.timestamp.strftime('%Y-%m-%dT%H:%M:%S+00:00') or ''
+        # OPDS atom:updated is defined as "the most recent instant in time
+        # when the entry was modified". Books.timestamp is the date added and
+        # never changes after import, so metadata and cover edits were
+        # invisible to OPDS sync clients. Use last_modified, which Calibre
+        # updates on every metadata or cover change; fall back to timestamp
+        # only if last_modified happens to be missing.
+        t = self.last_modified or self.timestamp
+        if t is None:
+            return ''
+        return t.strftime('%Y-%m-%dT%H:%M:%S+00:00') or ''
 
 
 class CustomColumns(Base):