mentor.getRecentMessages: last N unread messages from teams across all
of a mentor's assignments. Drives a Recent Messages card on /mentor.
applicant.getMentorConversationPreview: last 3 messages + unread count
for a given project. Drives a 'Conversation with [Mentor]' card on
/applicant — auto-hides when no mentor is assigned.
Both procedures use the existing MentorMessage(projectId, createdAt)
composite index — no new index needed.
Plan: docs/superpowers/plans/2026-04-28-pr6-multi-role-and-workspace-previews.md
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