fix: resolve 8 architecture flaws across download, playlist, and webhook

- Add --continue and --max-filesize to yt-dlp for download resilience
- Route thumbnail downloads through HTTP_PROXY
- Show per-video progress (i/N) with cancel for playlist downloads
- Run playlist asynchronously so cancel callbacks can be processed
- Guard handlePlaylistConfirm against concurrent active jobs
- Close Done channel in playlist entry lifecycle
- Add stepState TTL (30 min) to prevent stale states
- Wrap webhook server for graceful shutdown on SIGTERM
- Refactor formatBytes to loop-based implementation
- Show first line of stderr in user-facing error messages
- Fix format pointer reuse in playlist loop
- Add CreatedAt field to stepState for TTL tracking
- Add MaxFileSize and ProgressPrefix fields to DownloadJob

cmd/bot/webhook.go

@@ -5,6 +5,7 @@ import (
 	"log/slog"
 	"net/http"
 	"os"
+	"time"
 
 	tgbot "github.com/go-telegram/bot"
 )
@@ -23,10 +24,26 @@ func startWebhook(ctx context.Context, b *tgbot.Bot, webhookURL string) {
 	tlsCert := os.Getenv("BOT_TLS_CERT")
 	tlsKey := os.Getenv("BOT_TLS_KEY")
 
+	server := &http.Server{
+		Addr:    listenAddr,
+		Handler: b.WebhookHandler(),
+	}
+
+	// Shutdown HTTP server gracefully when context is cancelled
+	go func() {
+		<-ctx.Done()
+		slog.Info("shutting down webhook server")
+		shutdownCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		if err := server.Shutdown(shutdownCtx); err != nil {
+			slog.Error("webhook server shutdown", "error", err)
+		}
+	}()
+
 	if tlsCert != "" && tlsKey != "" {
 		slog.Info("starting HTTPS webhook", "addr", listenAddr)
 		go func() {
-			if err := http.ListenAndServeTLS(listenAddr, tlsCert, tlsKey, b.WebhookHandler()); err != nil {
+			if err := server.ListenAndServeTLS(tlsCert, tlsKey); err != nil && err != http.ErrServerClosed {
 				slog.Error("HTTPS server", "error", err)
 				os.Exit(1)
 			}
@@ -34,7 +51,7 @@ func startWebhook(ctx context.Context, b *tgbot.Bot, webhookURL string) {
 	} else {
 		slog.Info("starting HTTP webhook (TLS by reverse proxy)", "addr", listenAddr)
 		go func() {
-			if err := http.ListenAndServe(listenAddr, b.WebhookHandler()); err != nil {
+			if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
 				slog.Error("HTTP server", "error", err)
 				os.Exit(1)
 			}