feat(faq): add satisfaction rating with admin visibility

[kshethragna-mikkineni]

What changed

Adds a FAQ satisfaction rating system: users (guest or authenticated) rate
an FAQ's helpfulness on a 1–5 scale, ratings aggregate into satisfactionAvg
and satisfactionCount on the FAQ, and admins can view and sort by this
average in the admin FAQ table. Built by a 5-person team across the data
model, backend API, frontend rating UI, and admin surface.

Related issue

Closes #202

Type of change

• Feature

Area affected

• Backend (Express / Mongoose)
• Frontend (React / Vite)
• Admin / Train tab (/admin/*)
• Docs

CI verification

• cd apps/backend && npx tsc --noEmit exits 0
• cd apps/backend && npx vitest run — all tests pass
• cd apps/frontend && npx tsc --noEmit exits 0
• cd apps/frontend && npx vitest run — all tests pass
• pnpm run lint — 0 errors (152 warnings is the baseline)
• GitHub Actions green on the merge commit (CI, CodeQL, Build & Deploy)
• Tested with a real API hit or browser interaction if behaviour changed
• Tests added or updated for the change
• Single logical change — unrelated fixes noted in description, not fixed here
• Docs updated if route / API / env var / pipeline behaviour changed
• Rebased onto main, no merge commits

Notes for reviewer

This is a draft PR, opened early per repo convention, from a shared
branch with 5 contributors. Current status:

• Done: data model + aggregate helper, backend API (GET/POST satisfaction),
  frontend rating slider, admin sortable satisfactionAvg column
• In progress: backend Vitest tests, Playwright e2e test,
  docs/reference/openapi.yaml and docs/reference/database-schema.md updates
• Admin-side sorting is client-side only — no server index exists yet on
  satisfactionAvg; flagged as a possible follow-up, not blocking.
• Will request review and move out of draft once the above are complete and
  all CI verification boxes above are checked.