feat: Implement multi-tenancy with dynamic data source routing and introduce a database management UI.

frontend/admin/index.html

@@ -95,6 +95,14 @@
                 </svg>
                 Расписание занятий
             </a>
+            <a href="#" class="nav-item" data-tab="database">
+                <svg width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+                    <ellipse cx="12" cy="5" rx="9" ry="3"></ellipse>
+                    <path d="M21 12c0 1.66-4 3-9 3s-9-1.34-9-3"></path>
+                    <path d="M3 5v14c0 1.66 4 3 9 3s9-1.34 9-3V5"></path>
+                </svg>
+                База данных
+            </a>
         </nav>
         <div class="sidebar-footer">
             <button class="btn-logout" id="btn-logout">