Delete apiassets markdown note

Single free-form markdown note per asset.

Permissions reuse the existing READ_NOTES / WRITE_NOTES flags — callers who can already see comments on the asset can also see this note.

PUT semantics: idempotent upsert. The first PUT on an asset creates the note (201) and stamps author = request.user. Subsequent PUTs update only the content field (200) and do NOT refresh the author. Authorship records who originally wrote the note, not who last edited it — preserving history of who introduced the content. If a per-edit author trail becomes a requirement, add an updated_by FK; do not silently overwrite author on edit.