Document archive refactor

# Document archive refactor

Restructure how archived documents are stored and surfaced. A spec already exists in the engineering repo at `yawp-2.0/docs/work/document-archive.md` — port the relevant parts here so this becomes the canonical PM spec.

## Status note

The engineering-side spec was written before this PM repo existed. Action item: read `yawp-2.0/docs/work/document-archive.md`, lift the user-facing and product-relevant portions into this file, and leave engineering-only details (deploy steps, infra) in the engineering repo. After porting, link the engineering repo's version back to this file.

## Goals (rough)

- TBD on port. Core idea: archive lifecycle for documents that are no longer actively edited but should be retained for student/teacher access.

## Open questions

- [ ] Who owns the port (Bryant likely, since the engineering spec is theirs).
- [ ] Whether the engineering spec is still accurate or has drifted.

## Engineering handoff

Not ready. Spec needs to be ported and refreshed.