fix: don't purge connectivity/rtt alerts in purge_stale_alerts

These entries are set by the connection state machine, not by threshold
config, so they have no threshold entry and were being deleted on every
startup. Guard them explicitly so overdue/down alerts survive the purge.

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

hbd/server/threshold.py

@@ -1554,6 +1554,10 @@ class ThresholdChecker:
             configured = self.get_thresholds_for_host(hostname)
             stale = []
             for mp in host.alert_states:
+                # connectivity.* and rtt are managed by the connection state
+                # machine, not by threshold config — never purge them.
+                if mp == "rtt" or mp.startswith("connectivity."):
+                    continue
                 if self._find_threshold(configured, mp)[0] is not None:
                     continue
                 # Also match wildcard pool/partition thresholds (e.g. "zfs_monitor.*.status"