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/.env.example

@@ -1 +1,2 @@
 PUBLIC_AIA_API_BASE=https://api.azinstitute4autism.com
+PUBLIC_ALLOW_INDEXING=false