CVE-2026-123518: Security vulnerability

Vikunja before 2.7.1 contains an incorrect authorization vulnerability in TaskBucket.CanUpdate, which checks write access to the bucket's project and to the task separately but never verifies that both belong to the same project. Authenticated users with write access to two projects can move a task into another project's kanban bucket. This persists cross-project taskbuckets rows that count toward bucket limits and, for done buckets, toggle the task's done state.