Add Background Jobs dashboard for admin users

New admin page at /admin/jobs showing:
- PyPI cache job status (pending, in-progress, completed, failed)
- Failed task list with error details
- Retry individual packages or retry all failed
- Auto-refresh every 5 seconds (toggleable)
- Placeholder for future NPM cache jobs

Accessible from admin dropdown menu as "Background Jobs".

frontend/src/App.tsx

@@ -12,6 +12,7 @@ import APIKeysPage from './pages/APIKeysPage';
 import AdminUsersPage from './pages/AdminUsersPage';
 import AdminOIDCPage from './pages/AdminOIDCPage';
 import AdminCachePage from './pages/AdminCachePage';
+import AdminJobsPage from './pages/AdminJobsPage';
 import ProjectSettingsPage from './pages/ProjectSettingsPage';
 import TeamsPage from './pages/TeamsPage';
 import TeamDashboardPage from './pages/TeamDashboardPage';
@@ -52,6 +53,7 @@ function AppRoutes() {
                 <Route path="/admin/users" element={<AdminUsersPage />} />
                 <Route path="/admin/oidc" element={<AdminOIDCPage />} />
                 <Route path="/admin/cache" element={<AdminCachePage />} />
+                <Route path="/admin/jobs" element={<AdminJobsPage />} />
                 <Route path="/teams" element={<TeamsPage />} />
                 <Route path="/teams/:slug" element={<TeamDashboardPage />} />
                 <Route path="/teams/:slug/settings" element={<TeamSettingsPage />} />