CVE-2026-101272: fallo de gravedad baja en pretix

The mechanism binding files generated during a shredding process to the authentication method is not working correctly and the same session token is used for all token-based API users. This is similar to an issue fixed in 2026.7.1 but was missed while creating the fix. Since the files are referred to by randomly generated UUIDs and only exist for a day, there is virtually no risk, but it renders the added protection mechanism useless. As part of the fix, we have also improved hardening of some other related code areas.