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 (1-space indentation for headers, single trailing newline).
- Refactor `backend/models/User.js` to remove massive code duplication and adhere to Standard JS.
- Refactor `backend/server.js` to fix Deno linting and Standard JS issues (unused vars, quotes, semicolons).
- Correctly synchronize configuration files across root and public directories.
- Incorporate code review feedback for documentation prerequisites and thresholds.
Co-authored-by: OneFineStarstuff <87420139+OneFineStarstuff@users.noreply.github.com>