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/applicant.ts

@@ -2659,4 +2659,50 @@ export const applicantRouter = router({
 
       return { success: true, roundId: activePrs.roundId, roundName: activePrs.round.name }
     }),
+
+  /**
+   * Last N messages + unread count for the applicant's project mentor workspace.
+   * Drives the 'Conversation with [Mentor]' card on /applicant.
+   */
+  getMentorConversationPreview: protectedProcedure
+    .input(z.object({ projectId: z.string(), limit: z.number().min(1).max(10).default(3) }))
+    .query(async ({ ctx, input }) => {
+      const teamMembership = await ctx.prisma.teamMember.findFirst({
+        where: { projectId: input.projectId, userId: ctx.user.id },
+        select: { id: true },
+      })
+      if (!teamMembership) {
+        throw new TRPCError({
+          code: 'FORBIDDEN',
+          message: 'Not a team member of this project',
+        })
+      }
+
+      const assignment = await ctx.prisma.mentorAssignment.findUnique({
+        where: { projectId: input.projectId },
+        include: { mentor: { select: { id: true, name: true, email: true } } },
+      })
+
+      const [messages, unreadCount] = await Promise.all([
+        ctx.prisma.mentorMessage.findMany({
+          where: { projectId: input.projectId },
+          include: { sender: { select: { id: true, name: true, email: true } } },
+          orderBy: { createdAt: 'desc' },
+          take: input.limit,
+        }),
+        ctx.prisma.mentorMessage.count({
+          where: {
+            projectId: input.projectId,
+            senderId: { not: ctx.user.id },
+            isRead: false,
+          },
+        }),
+      ])
+
+      return {
+        mentor: assignment?.mentor ?? null,
+        messages: messages.reverse(),
+        unreadCount,
+      }
+    }),
 })