The background refresher was skipping token files with auth_method values
like 'IdC' or 'IDC' because the comparison was case-sensitive and only
matched lowercase 'idc'.
This fix normalizes the auth_method to lowercase before comparison in:
- token_repository.go: readTokenFile() when filtering tokens to refresh
- background_refresh.go: refreshSingle() when selecting refresh method
Fixes the issue where 'IdC' != 'idc' caused tokens to be skipped entirely.
