Users can only complete tasks if they are:
- Assigned to the task, OR
- An active member of the task's group, OR
- An admin (ADMIN_USERS env var)
Changes:
- TaskService.completeTask() now checks assignment, group membership,
and admin status before allowing completion
- Returns 'forbidden' status when unauthorized
- completar command handler shows appropriate error message
- Web path (loadTaskAndGating) gets admin override for consistency
- AdminService.isAdmin() made public for reuse
- Updated tests to respect new access control + 12 new test cases
borja