Where are project attributes like Operating System, Development Status, Programming Language, etc. stored in the database?
That information is encoded within the trove categories. This includes the trove_cat table which are all the trove categories and the trove_group_link table which are the trove categories associated with each group. For example, if you go to the summary page for a project on Sourceforge, and look at the link for (say) status, part of the URL has a query part form_cat=15 or some number.