feat: add sentinel dashboard roadmap and fix ci failures

- Create `docs/sentinel-dashboard-master-plan.md` and `docs/roadmap.md`.
- Fix Netlify `_headers` and `_redirects` formatting issues.
- Resolve Deno linting errors and reduce duplication in `backend/models/User.js`.
- Address linting issues in `backend/server.js` (quotes, semicolons, unused variables).
- Incorporate code review feedback for documentation consistency and prerequisites.

Co-authored-by: OneFineStarstuff <87420139+OneFineStarstuff@users.noreply.github.com>

Author: google-labs-jules[bot]