Adds mentor.dropAssignment mutation (mentor-only, ownership-checked, reason min 10 chars). Filters dropped MentorAssignment rows out of getMyProjects, getCandidates mentor count, getMentorPool, and getMenteeActivity so they no longer surface in the mentor or admin UI. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>