fix(seo): noindex staging builds by default

- add PUBLIC_ALLOW_INDEXING as an explicit production indexing opt-in
- emit robots meta noindex,nofollow unless indexing is enabled
- generate robots.txt from the same indexing flag
- document staging SEO behavior for migration preview hosts

www/public/robots.txt

@@ -1,3 +0,0 @@
-User-agent: *
-Allow: /
-Sitemap: https://www.azinstitute4autism.com/sitemap.xml