This change adds an admin role to GroupUser that allows group admins to:
1. Administer other users in the group
2. Change the group name
Changes include:
- Database migration to add isAdmin field to group_users table
- Updated GroupUser model to include isAdmin field
- Added isGroupAdmin policy utility function
- Updated group policy to allow group admins to update groups
- Added API endpoints for managing admin status
- Updated GroupUsersStore to handle admin functionality
- Added tests for the new functionality
codegen-sh[bot]