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/server/routers/mentor.ts

@@ -565,27 +565,45 @@ export const mentorRouter = router({
     }),
 
   /**
-   * Remove mentor assignment
+   * Remove mentor assignment.
+   *
+   * Multi-mentor (PR8): callers should pass `assignmentId` to target a
+   * specific co-mentor. Legacy callers passing only `projectId` get the
+   * most-recent assignment removed (kept for backward compatibility).
    */
   unassign: adminProcedure
-    .input(z.object({ projectId: z.string() }))
+    .input(
+      z
+        .object({
+          assignmentId: z.string().optional(),
+          projectId: z.string().optional(),
+        })
+        .refine((v) => !!v.assignmentId || !!v.projectId, {
+          message: 'Either assignmentId or projectId is required',
+        }),
+    )
     .mutation(async ({ ctx, input }) => {
-      // TODO(PR8 Task 8): admin UI should specify which mentor to drop when
-      // multiple are assigned. Legacy callers pass only projectId — we resolve
-      // to the most-recent assignment for backward compatibility.
-      const assignment = await ctx.prisma.mentorAssignment.findFirst({
-        where: { projectId: input.projectId },
-        orderBy: { assignedAt: 'desc' },
-        include: {
-          mentor: { select: { id: true, name: true } },
-          project: { select: { id: true, title: true } },
-        },
-      })
+      const assignment = input.assignmentId
+        ? await ctx.prisma.mentorAssignment.findUnique({
+            where: { id: input.assignmentId },
+            include: {
+              mentor: { select: { id: true, name: true } },
+              project: { select: { id: true, title: true } },
+            },
+          })
+        : await ctx.prisma.mentorAssignment.findFirst({
+            where: { projectId: input.projectId! },
+            orderBy: { assignedAt: 'desc' },
+            include: {
+              mentor: { select: { id: true, name: true } },
+              project: { select: { id: true, title: true } },
+            },
+          })
 
       if (!assignment) {
         throw new TRPCError({
           code: 'NOT_FOUND',
-          message: 'No mentor assignment found for this project',
+          message: 'No mentor assignment found',
         })
       }
 
@@ -602,7 +620,7 @@ export const mentorRouter = router({
         entityType: 'MentorAssignment',
         entityId: assignment.id,
         detailsJson: {
-          projectId: input.projectId,
+          projectId: assignment.project.id,
           projectTitle: assignment.project.title,
           mentorId: assignment.mentor.id,
           mentorName: assignment.mentor.name,