Add Groups management and split Admin navigation

- New backend: Group + GroupMembership models, schemas, CRUD router at
  /api/admin/groups (list, create, get detail, update, delete, add/remove members)
- New Alembic migration: groups and group_memberships tables
- Frontend: Admin sidebar item is now an expandable accordion with
  Users and Groups sub-items; AdminPage redirects to /admin/users;
  new AdminUsersPage and AdminGroupsPage with inline member management panel
- API client: 7 new group functions + TypeScript types

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

frontend/src/App.tsx

@@ -8,6 +8,8 @@ import DashboardPage from "./pages/DashboardPage";
 import ProfilePage from "./pages/ProfilePage";
 import AppsPage from "./pages/AppsPage";
 import AdminPage from "./pages/AdminPage";
+import AdminUsersPage from "./pages/AdminUsersPage";
+import AdminGroupsPage from "./pages/AdminGroupsPage";
 import DocumentsPage from "./pages/DocumentsPage";
 import DocumentAdminSettingsPage from "./pages/DocumentAdminSettingsPage";
 import AIAdminSettingsPage from "./pages/AIAdminSettingsPage";
@@ -51,6 +53,8 @@ export default function App() {
       />
       <Route path="/profile" element={<PrivateRoute><ProfilePage /></PrivateRoute>} />
       <Route path="/admin" element={<AdminRoute><AdminPage /></AdminRoute>} />
+      <Route path="/admin/users" element={<AdminRoute><AdminUsersPage /></AdminRoute>} />
+      <Route path="/admin/groups" element={<AdminRoute><AdminGroupsPage /></AdminRoute>} />
 
       {/* Catch-all */}
       <Route path="*" element={<Navigate to="/" replace />} />