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/dl/ytdlp.go

@@ -315,7 +315,7 @@ func (c *Client) StartDownload(job *domain.DownloadJob, downloadDir string) (<-c
 			<-progressDone
 			if err != nil {
 				job.Status = domain.StatusFailed
-				slog.Error("download failed", "url", job.URL, "error", err)
+				slog.Error("download failed", "url", job.URL, "error", err, "stderr", job.StderrLog)
 			} else {
 				job.Status = domain.StatusCompleted
 			}