Verify Cloudflare Access's signed token before trusting the proxy

The proxy sign-in believed Cf-Access-Authenticated-User-Email from any
address in trusted_proxies. On Tower that address is the Docker gateway,
so any container there could name itself anyone (docs/sso.md said as
much, and CLAUDE.md listed it as a known gap).

With [web] access_team and access_aud set, a proxied request must also
carry a Cf-Access-Jwt-Assertion that verifies against Cloudflare's keys
(RS256 only, this application's audience, the team's issuer, not
expired), and the name comes from its email claim. The keys are fetched
at start and again when a token names an unseen key, at most once a
minute, so made-up key ids cannot make every request a request to
Cloudflare. While the keys cannot be had, proxied sign-in is refused;
password and token sign-in are unaffected. Both settings empty, nothing
changes.

jsonwebtoken does the checking, on the aws-lc-rs backend already in the
tree through rustls. Tests sign with throwaway keys in tests/data: a
valid token, another app's audience, expired, a forged signature, HS256,
alg none, the refetch limit, and keys that cannot be fetched. Checked
live on a scratch daemon: the header alone and a forged token got 401,
the admin token still signed in.

vouched_name takes the peer and headers rather than the request: a
&Request held across the new await made the auth middleware's future
unsendable, as a body is not Sync.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/sso.md

@@ -172,9 +172,33 @@ itself, arrive under their own addresses and cannot set the header; the checks a
 sides. Never list a LAN address or range: anyone there could then send
 `Cf-Access-Authenticated-User-Email: rays@sdf1.net` and be you.
 
-**What ipx does not do:** it does not verify Cloudflare's signed `Cf-Access-Jwt-Assertion`. It
-trusts the hop. Verifying the signature would make the containers on Tower irrelevant to the
-boundary, and is the upgrade if that ever matters.
+**Unless the token is checked.** With `access_team` and `access_aud` set (next section), the
+header is not enough on its own: the request has to carry the token Cloudflare Access signed, and
+a container on Tower cannot make one.
+
+### Verifying Cloudflare's token
+
+Access adds `Cf-Access-Jwt-Assertion` to every request it forwards: a JWT naming the person,
+signed with keys only Cloudflare holds. With these two settings ipx checks it on every proxied
+request, and takes the name from its `email` claim.
+
+```toml
+[web]
+access_team = "<team>.cloudflareaccess.com" # Zero Trust → Settings: the team domain
+access_aud  = "…"                           # Access → Applications → ipodderx → Overview: Application Audience (AUD) Tag
+```
+
+ipx fetches the public keys from `https://<access_team>/cdn-cgi/access/certs` when it starts, and
+again when a token names a key it has not seen (Cloudflare rotates them every six weeks or so), at
+most once a minute. It checks the signature (RS256 only), that the audience is this application's
+tag, the issuer, and the expiry. Anything else is refused, and so is every proxied request while
+the keys cannot be fetched; password and token sign-in still work then.
+
+`trusted_header` and `trusted_proxies` still apply: the check is added to them, not put in their
+place.
+
+Check it: the busybox request under [Check it](#check-it), which sends the email header without a
+token from the Docker bridge, now gets `sign in`, and the site still signs you in through Authentik.
 
 **Turning it off:** clear `trusted_header` and restart. Proxy-made accounts stay, but nobody can sign
 in with them until they are given a password (`ipx user passwd <name>`).