Implement graceful shutdown for listeners, admin socket, eviction task, and ACME

- Replace handle.abort() for HTTPS server tasks with timeout-based join,
  allowing in-flight requests to drain before forceful shutdown
- Add shutdown_rx to start_admin_socket with tokio::select! for clean
  accept loop exit and Unix socket file cleanup on shutdown
- Add shutdown_rx to start_eviction_task with tokio::select! for
  cancellable eviction loop
- Add shutdown channel to spawn_acme_state for cancellable ACME state
  machine via tokio::select!
- Pass Arc<GracefulShutdown> through setup_tls to ACME state machine
- Move GracefulShutdown creation before admin socket and TLS setup
- Update integration test for new start_eviction_task signature

tests/integration_test.rs

@@ -244,10 +244,12 @@ async fn test_rate_limit_eviction_task() {
 
     limiter.check_and_consume(std::net::IpAddr::from([192, 168, 1, 1]));
 
+    let shutdown = Arc::new(reverse_proxy::shutdown::GracefulShutdown::new(30));
     let handle = reverse_proxy::rate_limit::start_eviction_task(
         limiter.clone(),
         Duration::from_millis(50),
         Duration::from_millis(100),
+        shutdown.subscribe(),
     );
 
     tokio::time::sleep(Duration::from_millis(200)).await;