docs/sso.md: the sign-in setup ipodderx.sdf1.net really runs

Authentik is Cloudflare Access's OpenID Connect identity provider, not
something in the request path, and the tunnel's requests reach ipx from
the content_default gateway, 192.168.16.1, not 127.0.0.1. The page is
rewritten from what was measured, with checks for both the trusted and
the refused path, and docs/history.md records every change made to get
there with how to undo it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TAC7sLVqfKmY6rsTLXzNgk

CLAUDE.md

@@ -17,6 +17,7 @@ Arcane project `content`: `/mnt/fast/arcane/projects/content/compose.yaml`. That
 | Database | `/mnt/user/ipodderx/state.db` | `/data/state.db` |
 | Downloads | `/mnt/user/ipodderx/downloads` | `/downloads` |
 | Web UI | `192.168.1.130:8099`, also `ipodderx.sdf1.net` via a Cloudflare tunnel | `0.0.0.0:8099` |
+| Sign-in via the tunnel | Cloudflare Access app `ipodderx`, with Authentik as its identity provider; see [docs/sso.md](docs/sso.md) | trusts `Cf-Access-Authenticated-User-Email` from `192.168.16.1`, the `content_default` gateway |
 
 Deploying a change is: build and push the image, then pull it and recreate the container.