feat: bullet and numbered lists, and undo across saves (engine + routes)

Lists: a paragraph can be marked as a bullet or numbered item. The list
definitions are written into word/numbering.xml and the part is registered
in the package when a document has none, so Word renders the list. A
document that already defines its own numbering keeps it - ours is added
beside it with unused ids. Consecutive items of the same kind share one
numId, so a numbered run counts 1, 2, 3. Clearing the list removes w:numPr.

Undo: POST /single-documents/:documentId/undo-edit restores the bytes of
the version preceding the current one and makes that current, so a save
can be stepped back without destroying later history.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018RCY6K4dNwdbo9gU3HQ9un