[#69625846] Adding server checks to postgres and backups

- Fortunately these checks should largely be token additions, as we already have criticals around `stagecraft_access_data_sets_health_check` on the backend app boxes, and a `keepalive` check hanging around.
- These should inform us something is wrong, but it's good to know that the box being down is the root cause.
- Now if only I could figure out how to remotely restart the box :-/
Merge pull request #373 from alphagov/add_server_checks_to_backups_and_postgres

[#69625846] Adding server checks to postgres and backups