fix(ssh): per-challenge SSH login + phew buffering/leak/timeout

Passwords failed on 10/16 challenges while state.json looked correct:
- only the 6 native GEMASTIK XVIII images provision 'ctfuser'; every imported
  XVI/XVII image does 'echo root:${PASSWORD} | chpasswd' and logs in as root.
  set_ssh_passwords() hardcoded ctfuser, so chpasswd set a password on an
  account nobody uses -> 'Permission denied' everywhere.
  Registry gains a per-challenge 'ssh_user'; chpasswd now targets the real
  login (and ctfuser/ctf when present) and reports failures loudly.
- phew checker: chall.py block-buffers stdout through the docker exec pipe
  (PYTHONUNBUFFERED now set) and leaks chall.py inside the container on
  timeout (26 orphans, container saturated) -> reaps the whole exec process
  group. Startup does a fresh Pailier keygen (~12 s) so crypto reads need
  _CRYPTO_TIMEOUT, not the 5 s prompt default.

Adds panel/verify_ssh_creds.py (proves the state->container binding from
inside via a real login), audit_ssh_users.sh, reset_runtime.sh.

receiver/challenges/xvi/Art.py

@@ -28,7 +28,7 @@ class Art(Challenge):
     def check(self):
         try:
             word = self.random_string(8)
-            url = f'http://localhost:{self.port}/art/{word}'
+            url = self.url(f'/art/{word}')
             r = requests.get(url, timeout=5)
             assert r.text == f'<iframe height="100%" width="100%" frameborder="0" src=https://asciified.thelicato.io/api/v2/ascii?text={word}></iframe>', 'Unexpected response'
             self.logger.info('Check passed for art')