SEO/Indexing for a CMS Behind a Host-Rewriting Reverse Proxy

Fix SEO and indexing for a CMS (Odoo, or any web app) served behind a Host-rewriting reverse proxy (nginx or Cloudflare), where the app sees a different internal host than the public domain and therefore emits the wrong robots.txt, sitemap, or canonical URLs. Use this skill whenever a site is not getting indexed, robots.txt says Disallow when it should not, the sitemap or canonical tags point at an internal host, an internal tenant host is itself a crawlable duplicate, or page/template edits do not render live, ESPECIALLY on the everjust.app / customdomain.ai Odoo platform. Covers the domain-mismatch robots+noindex bug, nginx sub_filter canonicalization, duplicate-host no…