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>

src/main.rs

@@ -1,3 +1,4 @@
+mod access;
 mod auth;
 mod config;
 mod db;
@@ -567,10 +568,16 @@ async fn start_web(
         tracing::warn!(bind, "web ui is reachable off this machine; the token is all that guards it");
     }
 
+    let access = Arc::new(access::Keys::default());
+    if let Some((team, _)) = ctx.cfg().web.access() {
+        let (access, ctx, team) = (access.clone(), ctx.clone(), team.to_owned());
+        tokio::spawn(async move { access.prefetch(&ctx.client, &team).await });
+    }
     let state = web::WebState {
         ctx: ctx.clone(),
         cmds: cmds.clone(),
         events: events.clone(),
+        access,
     };
     Ok(Some(tokio::spawn(async move {
         if let Err(e) = web::serve(state, &bind).await {