Cursor pagination now only applies when sorting by created_at.
Other sort fields (username, last_login_time, updated_at) will
not use cursor pagination to prevent data inconsistency.
Fixes: UserRepository.ListCursor() allowing sort fields that
don't match the cursor predicate.
- AssignRoles: wrap DeleteByUserID + BatchCreate in DB transaction (P1)
- GetUserRoles: use GetByIDs batch query instead of per-role GetByID loop (N+1 fix)
- ListAdmins: use GetByIDs batch query instead of per-user GetByID loop (N+1 fix)
- Add WithTx/DB methods to UserRoleRepository for transaction support
- Add GetByIDs to UserRepository (batch user lookup)
- Add .gitattributes to normalize line endings to LF (P2)
