feat(workspace): mentor + applicant message previews (§F.2)

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

src/server/routers/mentor.ts

@@ -1259,6 +1259,30 @@ export const mentorRouter = router({
       return messages
     }),
 
+  /**
+   * Recent unread messages from team members across all of the mentor's
+   * assignments. Drives the 'Recent Messages' card on /mentor.
+   */
+  getRecentMessages: mentorProcedure
+    .input(z.object({ limit: z.number().min(1).max(20).default(5) }).optional())
+    .query(async ({ ctx, input }) => {
+      const limit = input?.limit ?? 5
+      const unread = await ctx.prisma.mentorMessage.findMany({
+        where: {
+          senderId: { not: ctx.user.id },
+          isRead: false,
+          workspace: { mentorId: ctx.user.id },
+        },
+        include: {
+          sender: { select: { id: true, name: true, email: true } },
+          project: { select: { id: true, title: true } },
+        },
+        orderBy: { createdAt: 'desc' },
+        take: limit,
+      })
+      return { unread }
+    }),
+
   /**
    * List all mentor assignments (admin)
    */