Speakr: Open redirect in is_safe_url via parser mismatch on next parameter

Speakr is a personal, self-hosted web application designed for transcribing audio recordings. Prior to 0.8.20-alpha, the is_safe_url() helper used to validate post-login redirect targets applied urljoin(request.host_url, target) before parsing, while the controller passed the raw target to redirect(). A scheme-relative input such as ////evil.com resolved to a same-host URL during validation but was emitted verbatim in the Location header, where the browser interpreted it as a network-path-relative redirect to an attacker-controlled host. This vulnerability is fixed in 0.8.20-alpha.