arch+security: route all server contact through Pulse, harden SSH

Architecture:
- Remove direct subprocess ping from Gandalf; add PulseClient.ping()
  which runs the ping via the Pulse worker instead
- Remove standalone ping() function and subprocess import from monitor.py
- Add self.pulse alias to NetworkMonitor for convenience
- Both _process_ping_hosts() and snapshot builder now use self.pulse.ping()

Security:
- Change StrictHostKeyChecking=no → accept-new in both SSH command
  builders (monitor.py _ssh_batch, diagnose.py build_ssh_command).
  The Pulse worker's known_hosts is now authoritative; host keys are
  recorded on first connection and verified on all subsequent ones.
  MITM attacks after initial key exchange are now detectable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

diagnose.py

@@ -75,7 +75,7 @@ class DiagnosticsRunner:
         )
 
         return (
-            f'ssh -o StrictHostKeyChecking=no -o ConnectTimeout=5 '
+            f'ssh -o StrictHostKeyChecking=accept-new -o ConnectTimeout=5 '
             f'-o BatchMode=yes -o LogLevel=ERROR '
             f'-o ServerAliveInterval=10 -o ServerAliveCountMax=2 '
             f'root@{ip_q} \'{remote_cmd}\''

monitor.py

@@ -11,7 +11,6 @@ import json
 import logging
 import re
 import shlex
-import subprocess
 import time
 from datetime import datetime
 from typing import Dict, List, Optional
@@ -315,6 +314,14 @@ class PulseClient:
             return self.run_command(command, _retry=False)
         return None
 
+    def ping(self, ip: str, count: int = 3, timeout: int = 2) -> bool:
+        """Ping *ip* via the Pulse worker. Returns True if host responds."""
+        ip_q = shlex.quote(ip)
+        output = self.run_command(
+            f'ping -c {count} -W {timeout} {ip_q} >/dev/null 2>&1 && echo REACHABLE || echo UNREACHABLE'
+        )
+        return output is not None and output.strip() == 'REACHABLE'
+
 
 # --------------------------------------------------------------------------
 # Link stats collector (ethtool + Prometheus traffic metrics)
@@ -363,7 +370,7 @@ class LinkStatsCollector:
         shell_cmd = ' '.join(parts)
 
         ssh_cmd = (
-            f'ssh -o StrictHostKeyChecking=no -o ConnectTimeout=5 '
+            f'ssh -o StrictHostKeyChecking=accept-new -o ConnectTimeout=5 '
             f'-o BatchMode=yes -o LogLevel=ERROR '
             f'-o ServerAliveInterval=10 -o ServerAliveCountMax=2 '
             f'root@{ip} "{shell_cmd}"'
@@ -638,19 +645,6 @@ class LinkStatsCollector:
 # --------------------------------------------------------------------------
 # Helpers
 # --------------------------------------------------------------------------
-def ping(ip: str, count: int = 3, timeout: int = 2) -> bool:
-    try:
-        r = subprocess.run(
-            ['ping', '-c', str(count), '-W', str(timeout), ip],
-            stdout=subprocess.DEVNULL,
-            stderr=subprocess.DEVNULL,
-            timeout=30,
-        )
-        return r.returncode == 0
-    except Exception:
-        return False
-
-
 def _now_utc() -> str:
     return datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S UTC')
 
@@ -671,6 +665,7 @@ class NetworkMonitor:
         self.unifi = UnifiClient(self.cfg['unifi'])
         self.tickets = TicketClient(self.cfg.get('ticket_api', {}))
         self.link_stats = LinkStatsCollector(self.cfg, self.prom, self.unifi)
+        self.pulse = self.link_stats.pulse  # convenience alias
 
         mon = self.cfg.get('monitor', {})
         self.poll_interval = mon.get('poll_interval', 120)
@@ -838,7 +833,7 @@ class NetworkMonitor:
     def _process_ping_hosts(self, suppressions: list) -> None:
         for h in self.cfg.get('monitor', {}).get('ping_hosts', []):
             name, ip = h['name'], h['ip']
-            reachable = ping(ip)
+            reachable = self.pulse.ping(ip)
 
             if not reachable:
                 sup = db.check_suppressed(suppressions, 'host', name)
@@ -908,7 +903,7 @@ class NetworkMonitor:
 
         for h in self.cfg.get('monitor', {}).get('ping_hosts', []):
             name, ip = h['name'], h['ip']
-            reachable = ping(ip, count=1, timeout=2)
+            reachable = self.pulse.ping(ip, count=1, timeout=2)
             hosts[name] = {
                 'ip': ip,
                 'interfaces': {},