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/app/(mentor)/mentor/page.tsx

@@ -41,6 +41,7 @@ import {
 import { formatDateOnly } from '@/lib/utils'
 import { AnimatedCard } from '@/components/shared/animated-container'
 import { CountryDisplay } from '@/components/shared/country-display'
+import { RecentMessagesCard } from '@/components/mentor/recent-messages-card'
 
 // Status badge colors
 const statusColors: Record<string, 'default' | 'secondary' | 'destructive' | 'outline'> = {
@@ -117,6 +118,9 @@ export default function MentorDashboard() {
         </p>
       </div>
 
+      {/* Recent unread messages from teams */}
+      <RecentMessagesCard />
+
       {/* Stats */}
       <div className="grid gap-4 md:grid-cols-3">
         <AnimatedCard index={0}>