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/hub/systems/systems_test_helpers.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 
 	entities "github.com/henrygd/beszel/internal/entities/system"
+	"github.com/henrygd/beszel/internal/entities/systemd"
 	"github.com/pocketbase/pocketbase/core"
 )
 
@@ -134,3 +135,7 @@ func (s *System) CreateRecords(data *entities.CombinedData) (*core.Record, error
 	s.data = data
 	return s.createRecords(data)
 }
+
+func CreateSystemdStatsRecords(app core.App, data []*systemd.Service, systemId string) error {
+	return createSystemdStatsRecords(app, data, systemId)
+}