fix: set PYTHONUNBUFFERED=1 to force real-time progress output

yt-dlp is a Python script. When stderr is a pipe (not a TTY), Python
buffers output, causing progress updates to arrive in large chunks or
only at the end. Setting PYTHONUNBUFFERED=1 forces line-buffered output,
giving real-time progress.

internal/dl/ytdlp.go

@@ -265,6 +265,7 @@ func (c *Client) StartDownload(job *domain.DownloadJob, downloadDir string) (<-c
 	slog.Info("starting download", "url", job.URL, "format", formatID)
 
 	cmd := exec.Command(c.binaryPath, args...)
+	cmd.Env = append(os.Environ(), "PYTHONUNBUFFERED=1")
 
 	stderr, err := cmd.StderrPipe()
 	if err != nil {