Proxy a path to another app from AWS Amplify with custom rules

To serve extra routes (e.g. /u/*) from a separate app on a domain an Amplify app owns, add custom rules with status 200 and an external target (source /u/<*>, target https://other-host/u/<*>), placed before the SPA 404-200 catch-all; this works for WEB_COMPUTE (Next.js SSR) apps and needs no DNS change or edits to the hosted repo. Also proxy the other app asset prefix (/web-static/<*>). UpdateApp customRules replaces the whole list, so read the existing rules first and resend them. Separately: npm 404 on PUT during publish means you are logged out, not that the package is missing.