Sign out through the proxy when the proxy signed you in

Sign out cleared ipx's cookies and showed its password page, while
Cloudflare Access still vouched for the person: nothing was signed out,
and the page looked like the wrong login. /api/me now says, for someone
the proxy signed in, where to go instead ([web] sign_out_url, which is
/cdn-cgi/access/logout behind Access), and /login sends anyone the proxy
vouches for on to their feeds. The header check both use is one function.

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

src/config.rs

@@ -80,6 +80,10 @@ pub struct Web {
     pub trusted_proxies: Vec<String>,
     /// Create an account the first time the proxy vouches for a name it has not seen.
     pub auto_create_users: bool,
+    /// Where Sign out sends someone the proxy signed in. Signing out of ipx alone cannot stick
+    /// while the proxy still vouches for them, so this is the proxy's own sign-out:
+    /// `/cdn-cgi/access/logout` behind Cloudflare Access. Empty sends them to /login.
+    pub sign_out_url: String,
     /// Sign a session out after this long without a request.
     pub session_days: i64,
 }
@@ -93,6 +97,7 @@ impl Default for Web {
             trusted_header: String::new(),
             trusted_proxies: vec!["127.0.0.1".into(), "::1".into()],
             auto_create_users: true,
+            sign_out_url: String::new(),
             session_days: 30,
         }
     }