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/components/Layout.tsx

@@ -195,6 +195,17 @@ 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>