feat(admin): multi-mentor stacking UI + change-request inbox (PR8 Task 8)

- /admin/projects/[id]/mentor renders all co-mentors as a list with per-row
  Unassign (confirm dialog) and a stacking "Add a mentor" flow that no longer
  hides when at least one mentor is assigned. Candidates and AI suggestions
  filter out already-assigned mentors.
- Pending change-requests panel appears above the mentor list when there are
  open requests for the project, with per-card Mark Resolved / Dismiss actions
  routed through mentor.resolveChangeRequest (optional resolution note).
- MentoringRoundOverview gains a "Pending change requests" row showing the
  PENDING count across the program; the Review link deep-links to the first
  pending request's project mentor page.
- mentor.unassign now accepts { assignmentId } so the admin UI can target a
  specific co-mentor (legacy { projectId }-only callers still work and remove
  the most-recent assignment).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/admin/round/mentoring-round-overview.tsx

@@ -9,6 +9,7 @@ import {
   ArrowRight,
   Clock,
   FileText,
+  Inbox,
   MessageCircle,
   Target,
   UserCheck,
@@ -48,6 +49,10 @@ export function MentoringRoundOverview({ roundId }: Props) {
     { refetchInterval: 30_000 },
   )
   const { data: pool, isLoading: poolLoading } = trpc.mentor.getMentorPool.useQuery({})
+  const { data: pendingChangeRequests } = trpc.mentor.listChangeRequests.useQuery(
+    { status: 'PENDING' },
+    { refetchInterval: 30_000 },
+  )
 
   if (statsLoading || poolLoading) {
     return (
@@ -60,6 +65,15 @@ export function MentoringRoundOverview({ roundId }: Props) {
   }
   if (!stats || !pool) return null
 
+  const pendingCount = pendingChangeRequests?.length ?? 0
+  // If there's at least one pending request, deep-link directly into the
+  // first one's project (admins can resolve / view siblings from there).
+  // Otherwise the card stays static.
+  const firstPendingProjectId = pendingChangeRequests?.[0]?.project.id ?? null
+  const changeRequestsHref = firstPendingProjectId
+    ? `/admin/projects/${firstPendingProjectId}/mentor`
+    : null
+
   const requestedPct = stats.totalProjects
     ? Math.round((stats.requestedCount / stats.totalProjects) * 100)
     : 0
@@ -173,6 +187,42 @@ export function MentoringRoundOverview({ roundId }: Props) {
         </CardContent>
       </Card>
 
+      <Card
+        className={`md:col-span-2 xl:col-span-4 ${
+          pendingCount > 0 ? 'border-amber-300 dark:border-amber-700' : ''
+        }`}
+      >
+        <CardContent className="flex items-center justify-between py-4">
+          <div className="flex items-center gap-3">
+            <Inbox
+              className={`h-5 w-5 ${
+                pendingCount > 0 ? 'text-amber-600' : 'text-muted-foreground'
+              }`}
+            />
+            <div>
+              <div className="text-sm font-medium">Pending change requests</div>
+              <div className="text-muted-foreground text-xs">
+                Team members asking admin to swap a mentor
+              </div>
+            </div>
+          </div>
+          <div className="flex items-center gap-3">
+            <div className="text-2xl font-bold tabular-nums">{pendingCount}</div>
+            {changeRequestsHref ? (
+              <Link
+                href={changeRequestsHref}
+                className="text-muted-foreground hover:text-foreground inline-flex items-center text-xs"
+              >
+                Review
+                <ArrowRight className="ml-0.5 h-3 w-3" />
+              </Link>
+            ) : (
+              <span className="text-muted-foreground text-xs">All clear</span>
+            )}
+          </div>
+        </CardContent>
+      </Card>
+
       <Card className="md:col-span-2 xl:col-span-4">
         <CardHeader className="pb-2">
           <CardTitle className="text-sm">Workspace activity</CardTitle>