I think there will be a lot of regional variation. Here "civil servant" isn't used that often but I think of it as any non-elected government or government-like employees except for the police and the military. It's not restricted to administrative jobs, so could include jobs like mail carrier, social worker, inspector, nurses at a public hospital etc. That is, people who wouldn't be considered "bureaucrats".
Bureaucrat refers to someone doing administrative work. It's a word not often used to described oneself, because it can have a negative connotation (someone who is interested in paperwork and out of touch with the people).