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>
