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/components/mentor/recent-messages-card.tsx

@@ -0,0 +1,66 @@
+'use client'
+
+import Link from 'next/link'
+import { trpc } from '@/lib/trpc/client'
+import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card'
+import { Skeleton } from '@/components/ui/skeleton'
+import { MessageCircle } from 'lucide-react'
+
+function formatRelativePast(date: Date | string | null): string {
+  if (!date) return ''
+  const d = typeof date === 'string' ? new Date(date) : date
+  const ms = Date.now() - d.getTime()
+  const minutes = Math.floor(ms / 60_000)
+  const hours = Math.floor(ms / 3_600_000)
+  const days = Math.floor(ms / 86_400_000)
+  if (days > 0) return `${days}d ago`
+  if (hours > 0) return `${hours}h ago`
+  return `${Math.max(0, minutes)}m ago`
+}
+
+export function RecentMessagesCard() {
+  const { data, isLoading } = trpc.mentor.getRecentMessages.useQuery({ limit: 5 })
+
+  return (
+    <Card>
+      <CardHeader>
+        <CardTitle className="flex items-center gap-2 text-base">
+          <MessageCircle className="h-4 w-4" /> Recent Messages
+        </CardTitle>
+      </CardHeader>
+      <CardContent>
+        {isLoading ? (
+          <div className="space-y-2">
+            {[1, 2, 3].map((i) => (
+              <Skeleton key={i} className="h-12 w-full" />
+            ))}
+          </div>
+        ) : !data || data.unread.length === 0 ? (
+          <p className="text-muted-foreground text-sm">
+            No new messages. Your mentees will appear here when they reach out.
+          </p>
+        ) : (
+          <ul className="space-y-3">
+            {data.unread.map((m) => (
+              <li key={m.id}>
+                <Link
+                  href={`/mentor/workspace/${m.project.id}`}
+                  className="hover:bg-muted/40 block rounded-md border p-3"
+                >
+                  <div className="flex items-baseline justify-between gap-2">
+                    <div className="text-sm font-medium">{m.sender.name ?? m.sender.email}</div>
+                    <div className="text-muted-foreground text-xs">
+                      {formatRelativePast(m.createdAt as unknown as Date)}
+                    </div>
+                  </div>
+                  <div className="text-muted-foreground mt-0.5 text-xs">{m.project.title}</div>
+                  <div className="mt-1 line-clamp-2 text-sm">{m.message}</div>
+                </Link>
+              </li>
+            ))}
+          </ul>
+        )}
+      </CardContent>
+    </Card>
+  )
+}