feat(alerts): add alert for failed systemd services (#2173)

Adds a user-configurable "Failed Services" alert that notifies when any
tracked systemd service enters the failed state, and again when all services
recover.

---------

Signed-off-by: Martin Stenröse <martin@stenrose.se>
Co-authored-by: henrygd <hank@henrygd.me>

internal/alerts/alerts_system.go

@@ -37,7 +37,24 @@ func cpuStateAlertValue(name string, breakdown []float64) (float64, bool) {
 }
 
 func (am *AlertManager) HandleSystemAlerts(systemRecord *core.Record, data *system.CombinedData) error {
-	alerts := am.alertsCache.GetAlertsExcludingNames(systemRecord.Id, "Status")
+	// Systemd alerts are binary state, not numeric thresholds, so they're handled
+	// separately. They read their own state from the database and don't use data.
+	// Read the confirmed empty state from the record being saved instead of data:
+	// dashboard polling can replace the system's in-memory payload concurrently.
+	var currentInfo system.Info
+	confirmedEmptySnapshot := false
+	if err := systemRecord.UnmarshalJSONField("info", &currentInfo); err == nil {
+		confirmedEmptySnapshot = len(currentInfo.Services) > 0 && currentInfo.Services[0] == 0
+	}
+	if err := am.HandleSystemdAlerts(systemRecord, confirmedEmptySnapshot); err != nil {
+		am.hub.Logger().Error("Error handling systemd alerts", "err", err)
+	}
+
+	if data == nil {
+		return nil
+	}
+
+	alerts := am.alertsCache.GetAlertsExcludingNames(systemRecord.Id, "Status", alertNameSystemdFailed)
 	if len(alerts) == 0 {
 		return nil
 	}