feat: use notifiers on schedule (#362)

* fix potential memory leak with time.After

* add new background service to manage scheduled notifications

* update docs

* remove old js reference

* closes #278

* tidy

backend/internal/data/ent/schema/maintenance_entry.go

@@ -33,6 +33,8 @@ func (MaintenanceEntry) Fields() []ent.Field {
 			Optional(),
 		field.Float("cost").
 			Default(0.0),
+		field.Bool("reminders_enabled").
+			Default(false),
 	}
 }