The URL parser previously required the hostname to contain "civitai.com",
so CivitAI download links using the "civitai.red" domain were treated as
non-CivitAI URLs and rejected.
This extracts the host check into _is_civitai_host() and accepts both
"civitai.com" and "civitai.red" base domains, so resolve/parse flows
handle all CivitAI download links correctly.