* Add systemd_exporter Ansible role and Prometheus scrape config
- Create systemd_exporter role (download binary, create user, deploy service)
- Add scrape job for london-b:9558 and copenhagen-a:9558
- Add systemd_exporter_hosts inventory group
- Add stage 3b to deploy.yml
- Map role to deploy-on-merge scope
Closes PESO-120
* Fix line length lint violations in systemd_exporter tasks
* Fix var-naming lint: use systemd_exporter_ prefix for role variables
