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/S3.py

@@ -7,8 +7,6 @@ import os
 class S3(Challenge):
     flag_location = 'flags/s3.txt'
     history_location = 'history/s3.txt'
-    host = 'http://localhost:20000'
-
     def distribute(self, flag):
         try:
             with open(self.flag_location, 'w') as f:
@@ -29,11 +27,11 @@ class S3(Challenge):
             filename = self.random_string(8) + ".txt"
             content = self.random_string(64)
 
-            r = requests.post(f"http://localhost:{self.port}/upload", files={'file': (filename, content)})
+            r = requests.post(self.url(f'/upload'), files={'file': (filename, content)})
             assert r.status_code == 200
             assert r.text == f'Download <a href="/download?filename={filename}">here</a>'
 
-            r = requests.get(f"http://localhost:{self.port}/download?filename={filename}")
+            r = requests.get(self.url(f'/download?filename={filename}'))
             assert r.status_code == 200
             assert r.text == content