Add super admin panel and update README

- Admin account bootstrapped from ADMIN_EMAIL/ADMIN_PASSWORD env vars on startup
- Admin panel: list users, view content, reset passwords, disable/delete accounts
- is_admin and is_disabled columns on users table
- Disabled accounts blocked at login
- README updated with admin setup instructions and panel docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nginx/html/index.html

@@ -73,6 +73,9 @@
       <a href="#settings" class="nav-link" data-page="settings">
         <span class="nav-icon">&#9881;</span> Settings
       </a>
+      <a href="#admin" class="nav-link admin-only hidden" data-page="admin">
+        <span class="nav-icon">&#128272;</span> Admin
+      </a>
     </nav>
     <div class="sidebar-footer">
       <span id="sidebar-user" class="sidebar-user"></span>
@@ -277,6 +280,19 @@
         <span id="settings-status" class="settings-status"></span>
       </div>
     </section>
+    <!-- ADMIN -->
+    <section id="page-admin" class="page">
+      <div class="page-header">
+        <div>
+          <h1 class="page-title">Admin</h1>
+          <p class="page-subtitle">Manage user accounts</p>
+        </div>
+      </div>
+      <div id="admin-users-container">
+        <div class="empty-state">Loading users...</div>
+      </div>
+    </section>
+
   </main>
 
   <!-- MODALS -->