fix: correct ZFS pool status threshold operator and add per-metric grace

The default zfs_monitor.*.status threshold used operator '>' with warning=1,
so a DEGRADED pool (status=1) never alerted (1 > 1 is false) and a FAULTED
pool (status=2) only triggered WARNING instead of CRITICAL.

Fix the operator to '>=' in THRESHOLD_DEFAULTS and the example config.

Also adds a per-metric grace period override (ThresholdConfig.grace) so
individual thresholds can bypass or shorten the global grace delay. Alerts
with grace=0 fire immediately on state change rather than waiting for a
second collection cycle. Sets grace=0 on zfs_monitor.*.status so pool
degradation alerts fire on the first data report after the event.

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

hbd/server/config.py

@@ -113,8 +113,9 @@ THRESHOLD_DEFAULTS = {
                         'status': {
                             'warning': 1,
                             'critical': 2,
-                            'operator': '>',
+                            'operator': '>=',
                             'hysteresis': 0.0,
+                            'grace': 0,
                             'display': 'ZFS pool {pool_name} is {health}'
                         },
                         'capacity': {