fix(handlers): stop URL workers by closing the job channel

The old stop only set an atomic flag checked between jobs: a worker
blocked in recv() never woke (the channel was never closed), and queued
jobs were neither drained nor abandoned in a defined way despite the
"drains up to 256 jobs" comment. Now stop_url_workers sets the flag,
drops the sender so blocked recv() calls wake with None, and awaits the
worker JoinHandles (each finishes its in-flight job first). main awaits
it inside the existing 30s shutdown timeout.

crates/xmedia-bot/src/main.rs

@@ -185,7 +185,7 @@ async fn main() {
     const SHUTDOWN_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(30);
     let shutdown = async {
         let _ = stop_tx.send(true);
-        handlers::stop_url_workers();
+        handlers::stop_url_workers().await;
         if let Some(admin) = CONFIG.admin_ids.first() {
             let _ = bot.send_message(ChatId(*admin), "Shutting down...").await;
         }