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/Pasta.py

@@ -6,8 +6,6 @@ import requests
 class Pasta(Challenge):
     flag_location = 'flags/pasta.txt'
     history_location = 'history/pasta.txt'
-    host = "http://localhost:13000"
-
     def distribute(self, flag):
         try:
             with open(self.flag_location, 'w') as f:
@@ -25,7 +23,7 @@ class Pasta(Challenge):
 
     def check(self):
         try:
-            url = f'http://localhost:{self.port}'
+            url = self.url()
             username = f"checker-{self.random_string(8)}"
             pwd = self.random_string(12)
             flag = open(self.flag_location).read()