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/http.py

@@ -61,6 +61,13 @@ def _insert_threshold_metric(thresholds: dict, metric_path: str, values: dict) -
             except (TypeError, ValueError):
                 pass
 
+    grace = values.get("grace")
+    if grace is not None:
+        try:
+            cfg["grace"] = float(grace)
+        except (TypeError, ValueError):
+            pass
+
     count = values.get("count")
     if count is not None:
         try: