The public base URL for a request — shared by OAuth (OAuth endpoints, issuer) and LandingPlug (the MCP connection URL shown on the landing page).

Prefers a configured base URL (:persistent_term.put({:oauth_mcp_bridge, :base_url}, url) — set this if you have a stable issuer, e.g. a custom domain); otherwise derives from the request's forwarded headers, which is correct behind any TLS-terminating reverse proxy (Fly, Heroku, nginx, Cloudflare, ...).