feat: capture yt-dlp stderr on download failures for debugging

Add StderrLog field to DownloadJob and collect all stderr lines during
download. Log the captured stderr output when a download fails to help
diagnose exit code 2 errors (geo-restrictions, auth, etc.).

internal/domain/types.go

@@ -128,6 +128,7 @@ type DownloadJob struct {
 	FilePath       string
 	Title          string
 	QuotaApplied   bool
+	StderrLog      string
 	CancelCh       chan struct{}
 	Cmd            *exec.Cmd
 }