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.

panel/verify_ssh_creds.py

@@ -0,0 +1,68 @@
+#!/usr/bin/env python3
+"""Verify each team's SSH passwords actually work in the live containers.
+
+state.json can look perfect while the container holds a different password —
+set_ssh_passwords() races container boot and its failures are easy to miss.
+This proves the binding from the INSIDE (per the skill rule: never trust
+config, prove it with a real login).
+
+    python3 panel/verify_ssh_creds.py [teamIdx ...]
+"""
+import json
+import subprocess
+import sys
+from concurrent.futures import ThreadPoolExecutor
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent))
+import teams as orch
+
+def probe(user, pw, port, host="127.0.0.1"):
+    r = subprocess.run(
+        ["sshpass", "-p", pw, "ssh",
+         "-o", "StrictHostKeyChecking=no", "-o", "UserKnownHostsFile=/dev/null",
+         "-o", "ConnectTimeout=8", "-o", "LogLevel=ERROR",
+         "-p", str(port), f"{user}@{host}", "whoami; hostname"],
+        capture_output=True, text=True, timeout=30)
+    out = (r.stdout or "").strip().splitlines()
+    return (r.returncode == 0 and len(out) >= 2, out, (r.stderr or "").strip()[:80])
+
+def main():
+    want = [int(a) for a in sys.argv[1:]]
+    teams = [t for t in orch.list_teams() if not want or t["index"] in want]
+    for t in teams:
+        idx = t["index"]
+        names = [c["name"] for c in orch.enabled_challenges()]
+        jobs = []
+        for n in names:
+            if n not in (t.get("ports") or {}):
+                continue
+            jobs.append((n, t["ports"][n]["ssh"], t.get("chall_passwords", {}).get(n)))
+        ok = bad = 0
+        details = []
+        with ThreadPoolExecutor(max_workers=6) as ex:
+            futs = {ex.submit(probe, t.get("ssh_user", "ctfuser"), pw, port): n
+                    for n, port, pw in jobs if pw}
+            for fut, n in futs.items():
+                good, out, err = fut.result()
+                if good:
+                    ok += 1
+                    # hostname must be <challenge>_teamN — proves the binding
+                    details.append((n, out[1] if len(out) > 1 else "?"))
+                else:
+                    bad += 1
+                    details.append((n, f"FAIL {err}"))
+        print(f"team{idx} ({t.get('label')}): ssh {ok} ok / {bad} fail  (user={t.get('ssh_user')})")
+        for n, info in details:
+            if info == "FAIL" or info.startswith("FAIL"):
+                print(f"    {n}: {info}")
+        hosts = [i for n, i in details if not i.startswith("FAIL")]
+        mism = [(n, i) for n, i in details
+                if not i.startswith("FAIL") and not i.endswith(f"_team{idx}")]
+        if mism:
+            print(f"    !! hostname mismatch (not _team{idx}): {mism}")
+        else:
+            print(f"    all hostnames correct (e.g. {hosts[0] if hosts else '-'})")
+
+if __name__ == "__main__":
+    main()