feat: Mentees & Activity tab on /admin/mentors

Adds a project-centric ops view for mentor management:
- New mentor.getMenteeActivity tRPC procedure aggregates every project
  with wantsMentorship=true and derives a status (unassigned / assigned
  / active / stalled) from the latest message + file activity.
- /admin/mentors becomes a tabbed page: existing Mentor list +
  new Mentees & Activity table with status pills, search, and a
  per-row Assign/Open CTA linking to /admin/projects/[id]/mentor.
- Includes 2 unit tests covering classification + program scoping.

Also: ignore .remember/ (plugin scratch dir).

src/server/routers/mentor.ts

@@ -1072,6 +1072,125 @@ export const mentorRouter = router({
       }
     }),
 
+  /**
+   * Project-centric activity view: every project that wants mentorship,
+   * with assignment status, latest activity timestamps, and a derived
+   * status (unassigned / assigned / active / stalled).
+   * Drives the "Mentees & Activity" tab on /admin/mentors.
+   */
+  getMenteeActivity: adminProcedure
+    .input(z.object({ programId: z.string().optional() }))
+    .query(async ({ ctx, input }) => {
+      const projects = await ctx.prisma.project.findMany({
+        where: {
+          wantsMentorship: true,
+          ...(input.programId ? { programId: input.programId } : {}),
+        },
+        select: {
+          id: true,
+          title: true,
+          country: true,
+          status: true,
+          oceanIssue: true,
+          competitionCategory: true,
+          mentorAssignment: {
+            select: {
+              id: true,
+              method: true,
+              assignedAt: true,
+              completionStatus: true,
+              mentor: {
+                select: {
+                  id: true,
+                  name: true,
+                  email: true,
+                  maxAssignments: true,
+                  _count: { select: { mentorAssignments: true } },
+                },
+              },
+              messages: {
+                orderBy: { createdAt: 'desc' },
+                take: 1,
+                select: { createdAt: true },
+              },
+              files: {
+                orderBy: { createdAt: 'desc' },
+                take: 1,
+                select: { createdAt: true },
+              },
+              _count: { select: { messages: true, files: true } },
+            },
+          },
+          teamMembers: {
+            where: { role: 'LEAD' },
+            take: 1,
+            select: { user: { select: { name: true, email: true } } },
+          },
+        },
+        orderBy: { title: 'asc' },
+      })
+
+      const ACTIVE_WINDOW_MS = 7 * 86_400_000
+      const STALLED_WINDOW_MS = 14 * 86_400_000
+      const now = Date.now()
+
+      const totals = { unassigned: 0, assigned: 0, active: 0, stalled: 0 }
+
+      const rows = projects.map((p) => {
+        const ma = p.mentorAssignment
+        const lastMessageAt = ma?.messages[0]?.createdAt ?? null
+        const lastFileAt = ma?.files[0]?.createdAt ?? null
+        const lastActivityAt = [lastMessageAt, lastFileAt]
+          .filter((d): d is Date => d != null)
+          .sort((a, b) => b.getTime() - a.getTime())[0] ?? null
+
+        let status: 'unassigned' | 'assigned' | 'active' | 'stalled'
+        if (!ma) {
+          status = 'unassigned'
+        } else if (lastActivityAt && now - lastActivityAt.getTime() <= ACTIVE_WINDOW_MS) {
+          status = 'active'
+        } else {
+          const referenceTime = lastActivityAt ?? ma.assignedAt
+          const elapsed = now - referenceTime.getTime()
+          status = elapsed > STALLED_WINDOW_MS ? 'stalled' : 'assigned'
+        }
+        totals[status]++
+
+        const teamLead = p.teamMembers[0]?.user ?? null
+
+        return {
+          project: {
+            id: p.id,
+            title: p.title,
+            country: p.country,
+            status: p.status,
+            oceanIssue: p.oceanIssue,
+            competitionCategory: p.competitionCategory,
+          },
+          teamLead: teamLead ? { name: teamLead.name, email: teamLead.email } : null,
+          mentor: ma?.mentor
+            ? {
+                id: ma.mentor.id,
+                name: ma.mentor.name,
+                email: ma.mentor.email,
+                currentLoad: ma.mentor._count.mentorAssignments,
+                maxAssignments: ma.mentor.maxAssignments,
+              }
+            : null,
+          assignmentMethod: ma?.method ?? null,
+          assignedAt: ma?.assignedAt ?? null,
+          lastMessageAt,
+          lastFileAt,
+          lastActivityAt,
+          messageCount: ma?._count.messages ?? 0,
+          fileCount: ma?._count.files ?? 0,
+          status,
+        }
+      })
+
+      return { rows, totals }
+    }),
+
   /**
    * Get mentor's assigned projects
    */