fix: LLDP input validation, mgmt_ip early validation, poll timer cleanup, monitor backoff

- app.py: validate server_name from LLDP with fullmatch before use in logs/lookups (prevents log injection)
- app.py: validate each mgmt_ip candidate before assigning host_ip (avoids assigning non-IP string that then fails later check)
- app.py: log actual exception in link_stats JSON parse error
- inspector.html: clear _diagPollTimer in closePanel() so timer doesn't orphan when panel is closed mid-poll
- monitor.py: sleep 30s after a monitor loop exception before resuming normal poll interval

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

templates/inspector.html

@@ -231,6 +231,7 @@ function selectPort(el) {
 }
 
 function closePanel() {
+  if (_diagPollTimer) { clearInterval(_diagPollTimer); _diagPollTimer = null; }
   document.getElementById('inspector-panel').classList.remove('open');
   document.querySelectorAll('.switch-port-block.selected')
     .forEach(el => el.classList.remove('selected'));