The four candidates for Oakland mayor talked about the city budget, economic revitalization and other issues at a voter forum Saturday at City Hall, but the discussion kept returning to public safety.  Around 40 people attended the forum, which was organized with the League of Women Voters of Oakland. First-term Mayor Barbara Lee will face challengers Brenda Grisham, a business owner and longtime community advocate; Mindy Pechenuk, an educator and researcher; and Julius Robinson, a community investment executive, in the…