www: _/text/config/raw Last-Modified: is mm->created_at

www: _/text/config/raw Last-Modified: is mm->created_at

This allows IMAP mirrors to keep UIDVALIDITY synchronized (and
"LIST ACTIVE.TIMES" in NNTP).  "lei add-external --mirror" will
automatically set it, as will the combination of
public-inbox-clone + public-inbox-index.

This avoids the need for extra endpoints or config entries,
at least...