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).

amplify.yml

@@ -0,0 +1,20 @@
+version: 1
+applications:
+  - frontend:
+      phases:
+        preBuild:
+          commands:
+            - nvm use $(cat .nvmrc) || nvm install $(cat .nvmrc)
+            - npm ci --cache .npm --prefer-offline
+        build:
+          commands:
+            - npm run build
+      artifacts:
+        baseDirectory: .next
+        files:
+          - '**/*'
+      cache:
+        paths:
+          - node_modules/**/*
+          - .next/cache/**/*
+          - .npm/**/*