Permanently-blank Series rows heal on the next due cycle: the chapter-page
body already in hand is scanned for a Cover, bytes are stored, and the wire
address is set. A non-blank Cover is never replaced. Failures log against the
Series, leave the chapter poll alone, and retry on the next due cycle with no
separate queue. Kagane source-URL prefetch now lands on the same
content-addressed SetSeriesCover path as every other Site.
sulthan