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>

frontend/admin.html

@@ -919,7 +919,10 @@
           <svg class="adm-chev" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round"><polyline points="6 9 12 15 18 9"/></svg>
         </button>
         <div class="admin-nav-body">
-          <button class="admin-nav-item active" data-tab="stats" onclick="switchTab(this)">
+          <button class="admin-nav-item active" data-tab="overview" onclick="switchTab(this)">
+            <i data-lucide="layout-dashboard" style="width:15px;height:15px"></i> Обзор
+          </button>
+          <button class="admin-nav-item" data-tab="stats" onclick="switchTab(this)">
             <i data-lucide="bar-chart-2" style="width:15px;height:15px"></i> Статистика
           </button>
           <button class="admin-nav-item" data-tab="sessions" onclick="switchTab(this)">
@@ -1016,8 +1019,13 @@
     </nav>
     <div class="admin-main">
 
+    <!-- ── Обзор (Phase 3) ── -->
+    <div class="tab-pane active" id="tab-overview">
+      <div id="overview-content"><div class="spinner"></div></div>
+    </div>
+
     <!-- ── Статистика ── -->
-    <div class="tab-pane active" id="tab-stats">
+    <div class="tab-pane" id="tab-stats">
       <div class="section-title">Общая статистика</div>
       <div class="stats-grid" id="stats-grid"><div class="spinner"></div></div>
       <div class="section-title">По предметам</div>
@@ -1983,6 +1991,7 @@
   <script src="/js/sidebar.js"></script>
   <script src="/js/admin/router.js"></script>
   <script src="/js/admin/_shared.js"></script>
+  <script src="/js/admin/sections/overview.js"></script>
   <script src="/js/admin/sections/stats.js"></script>
   <script src="/js/admin/sections/sublog.js"></script>
   <script src="/js/admin/sections/sims.js"></script>