attach-1: when the same attachmentId was referenced by more than one page
in a duplicated subtree, the per-attachmentId map held only a single copy
entry, so the last page processed clobbered the others. The downstream
ownership guard (`attachment.pageId !== oldPageId`) then matched at most one
page and skipped the lone DB row entirely: no blob copied, no new row, every
copy's image 404'd. Key the map to a list of entries and copy one blob/row
per referencing page; drop the now-incorrect ownership guard.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
c919d4f636