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

internal/handler/handler.go

@@ -464,21 +464,15 @@ func formatBytes(bytes int64) string {
 	if bytes < unit {
 		return strconv.FormatInt(bytes, 10) + "B"
 	}
-	div, exp := int64(unit), 0
-	for n := bytes / unit; n >= unit; n /= unit {
-		div *= unit
-		exp++
-	}
-	switch exp {
-	case 0:
-		return fmt.Sprintf("%.1fKB", float64(bytes)/float64(div))
-	case 1:
-		return fmt.Sprintf("%.1fMB", float64(bytes)/float64(div))
-	case 2:
-		return fmt.Sprintf("%.1fGB", float64(bytes)/float64(div))
-	default:
-		return fmt.Sprintf("%.1fTB", float64(bytes)/float64(div))
+	units := []string{"KB", "MB", "GB", "TB"}
+	size := float64(bytes)
+	for _, u := range units {
+		size /= unit
+		if size < unit {
+			return fmt.Sprintf("%.1f%s", size, u)
+		}
 	}
+	return fmt.Sprintf("%.1fTB", size)
 }
 
 func (h *Handler) generateJobID() string {