feat: generic ping_monitor thresholds; round RTT to nearest ms

- threshold.py: add _find_threshold() with suffix fallback so thresholds
  like ping_monitor.rtt_avg match ping_monitor.8_8_8_8_rtt_avg etc.;
  each pinged host keeps its own alert state
- hbdclass.py: format RTT as integer ms (round())
- live.html: JS RTT display rounded to nearest ms (Math.round)

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

hbd/server/templates/live.html

@@ -408,7 +408,7 @@
         );
         if (data.connections[i].state == "up") {
           state = '<span class="state-up">up</span>';
-          latency = Number.parseFloat(data.connections[i].rtts[0]).toFixed(2);
+          latency = String(Math.round(Number.parseFloat(data.connections[i].rtts[0])));
         } else {
           if (data.connections[i].state == "unknown") {
             state = "";