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/migrations/1_add_systemd_failed_alert.go

@@ -0,0 +1,44 @@
+package migrations
+
+import (
+	"errors"
+	"slices"
+
+	"github.com/pocketbase/pocketbase/core"
+	m "github.com/pocketbase/pocketbase/migrations"
+)
+
+// alertNameSystemdFailed is the alerts.name select value for the
+// "failed systemd services" alert type.
+const alertNameSystemdFailed = "SystemdFailed"
+
+func init() {
+	m.Register(func(app core.App) error {
+		return updateAlertNameValues(app, func(values []string) []string {
+			if slices.Contains(values, alertNameSystemdFailed) {
+				return values
+			}
+			return append(values, alertNameSystemdFailed)
+		})
+	}, func(app core.App) error {
+		return updateAlertNameValues(app, func(values []string) []string {
+			return slices.DeleteFunc(values, func(value string) bool {
+				return value == alertNameSystemdFailed
+			})
+		})
+	})
+}
+
+// updateAlertNameValues applies fn to the values of the alerts.name select field and saves the collection.
+func updateAlertNameValues(app core.App, fn func([]string) []string) error {
+	collection, err := app.FindCollectionByNameOrId("alerts")
+	if err != nil {
+		return err
+	}
+	field, ok := collection.Fields.GetByName("name").(*core.SelectField)
+	if !ok {
+		return errors.New("alerts.name is not a select field")
+	}
+	field.Values = fn(field.Values)
+	return app.Save(collection)
+}