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/components/Layout.tsx

@@ -195,17 +195,6 @@ function Layout({ children }: LayoutProps) {
                           </svg>
                           Cache Management
                         </NavLink>
-                        <NavLink
-                          to="/admin/jobs"
-                          className="user-menu-item"
-                          onClick={() => setShowUserMenu(false)}
-                        >
-                          <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2">
-                            <rect x="2" y="7" width="20" height="14" rx="2" ry="2"/>
-                            <path d="M16 21V5a2 2 0 0 0-2-2h-4a2 2 0 0 0-2 2v16"/>
-                          </svg>
-                          Background Jobs
-                        </NavLink>
                       </>
                     )}
                     <div className="user-menu-divider"></div>