AWS Amplify Hosting deploy prep

- amplify.yml: build spec (preBuild npm ci with offline cache, build
  next build, artifacts at .next/**, cache node_modules/.next-cache/.npm).
- .nvmrc: pin Node 20 so Amplify uses the same runtime as local.
- AMPLIFY_DEPLOY.md: first-time walkthrough covering AWS-side setup
  (create app, connect GitHub via OAuth/App, branch/auto-detect),
  environment variable table with secret-flag guidance, smoke-test via
  /healthz and /api/health, optional custom domain + per-PR previews,
  cost estimate, and operational notes (cold starts, no static
  egress IPs, CloudWatch logs, secret rotation).
- README deploy section: now points at both AMPLIFY_DEPLOY.md and the
  existing DEPLOYMENT.md (Render).

README.md

@@ -105,11 +105,11 @@ works while in stub mode. The report is at `/report?cid=1&cs=anything`.
 
 ## Deploy
 
-The repo ships with `render.yaml` for [Render](https://render.com) and a
-detailed `DEPLOYMENT.md` covering Render specifically. The app is a
+The repo ships with `amplify.yml` for [AWS Amplify Hosting](https://aws.amazon.com/amplify/)
+(see `AMPLIFY_DEPLOY.md` for a first-time walkthrough) and `render.yaml`
+for [Render](https://render.com) (see `DEPLOYMENT.md`). The app is a
 standard Next.js 16 App Router project and runs anywhere Node 20+ can
-run `next start` — Vercel, AWS Amplify Hosting, App Runner, Fly,
-self-hosted, etc.
+run `next start` — Vercel, App Runner, Fly, self-hosted, etc.
 
 Build + start: