CVE-2026-43978: wger: Privilege escalation via trainer-login session chaining allows gym trainer to impersonate gym manager

A gym trainer can escalate their session to any higher-privileged account (gym manager, general manager) by chaining two calls to the trainer-login endpoint. Once a trainer performs a legitimate switch into a low-privileged user, the session flag trainer.identity is set and this flag alone bypasses the permission check on all subsequent trainer-login calls, allowing the trainer to hop into any account including gym managers.