test: cover session tokens, and run the suite in make check

auth.rs was the largest untested surface and the one where a mistake is
worth the most: the tests pin the round trip, expiry, secret rotation,
forged expiries, flipped signature bytes, malformed input, and that
credentials have to match in full rather than by prefix.

Also documents autosave, :export! and the security headers in the README.

src/auth.rs

@@ -94,3 +94,77 @@ impl FromRequestParts<AppState> for Session {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{credentials_match, issue_token, verify_token};
+    use crate::config::Config;
+
+    fn config(session_days: i64) -> Config {
+        Config {
+            username: "narl".into(),
+            password: "hunter2".into(),
+            secret: b"a-secret-of-at-least-16-bytes".to_vec(),
+            bind: "127.0.0.1:0".parse().unwrap(),
+            data_dir: ".".into(),
+            session_days,
+            max_upload_bytes: 1,
+            secure_cookie: false,
+        }
+    }
+
+    #[test]
+    fn a_fresh_token_verifies() {
+        let config = config(30);
+        assert!(verify_token(&config, &issue_token(&config)));
+    }
+
+    #[test]
+    fn an_expired_token_does_not() {
+        let config = config(-1);
+        assert!(!verify_token(&config, &issue_token(&config)));
+    }
+
+    #[test]
+    fn rotating_the_secret_invalidates_every_session() {
+        let token = issue_token(&config(30));
+        let mut rotated = config(30);
+        rotated.secret = b"a-different-secret-entirely".to_vec();
+        assert!(!verify_token(&rotated, &token));
+    }
+
+    #[test]
+    fn a_tampered_token_does_not_verify() {
+        let config = config(30);
+        let token = issue_token(&config);
+        let (payload, signature) = token.split_once('.').unwrap();
+
+        // A far-future expiry the holder signed themselves.
+        let forged = base64::Engine::encode(&super::B64, "99999999999");
+        assert!(!verify_token(&config, &format!("{forged}.{signature}")));
+
+        // The real expiry with the last byte of the signature flipped.
+        let mut bytes = signature.as_bytes().to_vec();
+        bytes[0] = if bytes[0] == b'A' { b'B' } else { b'A' };
+        let flipped = String::from_utf8(bytes).unwrap();
+        assert!(!verify_token(&config, &format!("{payload}.{flipped}")));
+    }
+
+    #[test]
+    fn malformed_tokens_are_rejected_rather_than_panicking() {
+        let config = config(30);
+        for token in ["", ".", "no-dot", "!!!.!!!", "a.b.c", &".".repeat(100)] {
+            assert!(!verify_token(&config, token), "{token:?}");
+        }
+    }
+
+    #[test]
+    fn credentials_must_match_exactly() {
+        let config = config(30);
+        assert!(credentials_match(&config, "narl", "hunter2"));
+        assert!(!credentials_match(&config, "narl", "hunter"));
+        assert!(!credentials_match(&config, "narl", "hunter22"));
+        assert!(!credentials_match(&config, "nar", "hunter2"));
+        assert!(!credentials_match(&config, "", ""));
+    }
+}