docs: draft document-centric list and history RFC

[Frankie-Xu]

Unifying RFC for #232 and #231. No API or code change.

Summary

• Add docs/design/document-centric-list-history.md.
• Restates suguanYang's ownership rules: namespace is client-owned, jobs are the processing ledger, documents are the corpus resource.
• Notes what already exists on main: GET /documents?namespace=, POST /documents/{id}/archive, GET /jobs/{id}.
• The only remaining API gap called out is document-scoped history (GET /documents/{id}/jobs or /revisions).
• Explicitly not a competing implementation of feat: list namespaces endpoint #232/feat(jobs): filter job listing by namespace and add job deletion #231; those PRs stay with @gdccyuen and @SusannaShu.

Verification

• Docs-only.
• Rechecked apps/api/app/api/v1/routes/documents.py and jobs.py on 9489fa2c.

Deployment Notes

• No runtime, env, or migration change

Checklist

• Tests were added or updated when behavior changed — N/A
• Public docs, examples, or OpenAPI contracts were updated when needed — RFC draft only
• Database migrations are idempotent and safe to deploy — N/A
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change — none