Remove redundant nav entries; parallelize cleanup file checks

- Navbar: drop the Users link (already reachable from Admin's quick-links)
- Admin quick-links: drop Settings (already reachable from the navbar
  for every user)
- library_cleanup.py: parallelize the missing-file stat checks across a
  small thread pool instead of one file at a time — sequential NAS round
  trips were taking minutes for a library of any real size, even after
  moving them off the event loop.

frontend/src/components/Navbar.jsx

@@ -35,7 +35,6 @@ export const Navbar = () => {
               <NavLink to="/music" className={linkClass} data-testid="nav-music">Music</NavLink>
               <NavLink to="/requests" className={linkClass} data-testid="nav-requests">Wishlist</NavLink>
               {user.is_admin && <NavLink to="/admin" className={linkClass} data-testid="nav-admin">Admin</NavLink>}
-              {user.is_admin && <NavLink to="/admin/users" className={linkClass} data-testid="nav-users">Users</NavLink>}
             </nav>
           )}
         </div>