Remove proactive PyPI dependency caching feature

The background task queue for proactively caching package dependencies was
causing server instability and unnecessary growth. The PyPI proxy now only
caches packages on-demand when users request them.

Removed:
- PyPI cache worker (background task queue and worker pool)
- PyPICacheTask model and related database schema
- Cache management API endpoints (/pypi/cache/*)
- Background Jobs admin dashboard
- Dependency extraction and queueing logic

Kept:
- On-demand package caching (still works when users request packages)
- Async httpx for non-blocking downloads (prevents health check failures)
- URL-based cache lookups for deduplication

frontend/src/App.tsx

@@ -12,7 +12,6 @@ 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';
@@ -53,7 +52,6 @@ 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 />} />