The authelia vhost intercepts upstream errors and serves a friendly
"SSO unavailable" page. The `=` form of `error_page` takes its status
from the redirected location, and that location serves a file -- so the
page is returned as **200**.
That is right for a human typing the URL and wrong for every machine
caller, all of which reach authelia through this same vhost by name:
- `/api/authz/auth-request` -- nginx `auth_request` treats any 2xx as
success, so a down authelia means access GRANTED
- `/api/oidc/introspection` -- a token check answering 200
- `/api/oidc/token`, `/.well-known/openid-configuration` -- clients
parsing an HTML error page as their JSON document
Routes `/api/` and `/.well-known/` without the interception. A longer
prefix wins over `/`, and the intercept directives live inside the `/`
location rather than at server level, so they do not reach the new ones.
Split by AUDIENCE rather than by an enumerated path list: a human still
gets the page, and every machine caller -- including the login page's own
XHR, and any endpoint added later -- gets the real status.
Measured against a real nginx with a dead upstream, both arms: machine
paths return 502 where they returned 200+HTML, a subrequest through the
new prefix denies (matching a direct port dial) where through `/` it
served the protected content, and the browser control confirms the
friendly page survives. URI preservation checked separately against a
live echo upstream -- `proxy_pass` with no URI part passes the full
original path.
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