fix: reduce default hysteresis 10%→2%; show recovery threshold in alerts UI

The 10% default hysteresis created an unreasonably wide recovery band:
a 95% threshold would only clear once the value dropped below 85.5%,
causing alerts to linger long after the metric was well below the
trigger level.

Change default hysteresis to 2% across all threshold parsers (plugin
metrics, partitions, RTT). For a 95% threshold, recovery is now at
93.1% instead of 85.5%.

Add AlertState.hysteresis field (set on every check, cleared on OK) and
expose recovery_threshold in to_dict() so the Alerts dashboard can
display "recovers < 93.1" alongside the trigger threshold, making the
hysteresis band visible to the user. Pickle backward-compatible via
__setstate__.

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

hbd/server/templates/alerts.html

@@ -405,6 +405,10 @@
         } else if (alert.threshold_value !== undefined && alert.threshold_value !== null && alert.operator) {
           valueText += ` <span class="threshold-info">(threshold: ${alert.operator} ${formatValue(alert.threshold_value)})</span>`;
         }
+        if (alert.recovery_threshold !== undefined && alert.recovery_threshold !== null) {
+          const recOp = (alert.operator === '>' || alert.operator === '>=') ? '<' : '>';
+          valueText += ` <span class="threshold-info" style="color:#888">(recovers ${recOp} ${formatValue(alert.recovery_threshold)})</span>`;
+        }
         
         // Build actions section
         let actionsHtml = '';