In single-node setups, guest alerts had Instance == Node, causing
reevaluateActiveAlertsLocked to evaluate them against NodeDefaults
instead of GuestDefaults. Setting guest memory threshold to 0 (disabled)
wouldn't clear existing guest alerts because they were being kept alive
by the still-enabled node memory threshold.
- Add resourceID colon check to distinguish guest IDs (instance:node:vmid)
from node IDs (instance-node) in reevaluateActiveAlertsLocked
- Clear stale alerts in checkMetric when threshold is nil or disabled
- Skip hysteresis validation for disabled thresholds (Trigger <= 0)
- Fix frontend tooltip: "0" not "-1" disables a threshold
