We need to get rid of usages of `threads.default_role` throughout the codebase so we can eventually drop the column.
This diff handles the usage in `thread-fetchers.js`
Depends on D9979
Addresses part of [[ https://linear.app/comm/issue/ENG-5833/update-code-that-uses-default-role-from-mariadb | ENG-5833 ]]