feat(admin): phase 3 — dashboard #overview landing

GET /api/admin/overview returns 24h digest (~0.08ms/call).

- adminController.getOverview: 7 prepared statements (users 24h, sessions 24h, active users, classes count, failed sessions, banned this week, top-5 sessions)

- new section frontend/js/admin/sections/overview.js (~205L): bento-grid cards, alerts (only when >0), top-5 table, quick-links

- nav-item + tab-pane reordered: #overview is now default; #stats remains routable

Auth: admin-only (inside requireRole('admin') block, sibling of /stats).

Backward compat: all 13 existing routes unchanged.

Known follow-ups (post-merge polish):

- activeClasses counts all (label could be 'Всего классов')

- failedSessions24h includes in_progress (could tighten to abandoned only)

- topSessions24h drops NULL-score completed rows

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

backend/src/routes/admin.js

@@ -14,6 +14,7 @@ router.patch('/free-student-features',        requireRole('admin'), ctrl.updateF
 router.use(requireRole('admin'));
 
 router.get('/stats',                  ctrl.getStats);
+router.get('/overview',               ctrl.getOverview);
 router.get('/users',                  ctrl.getUsers);
 router.patch('/users/:id/role',       ctrl.updateRole);
 router.get('/users/:id/sessions',     ctrl.getUserSessions);