fix(downloader): drain yt-dlp pipes before Wait to avoid output loss

exec.Cmd.Wait closes the stdout/stderr pipes on process exit, which
discards buffered output the scanners have not read yet - a race that
dropped progress lines and stderr tails on CI. Readers now drain to
EOF first, then Wait reaps the process and returns the exit code.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

pkg/downloader/ytdlp/ytdlp.go

@@ -208,8 +208,10 @@ func (p *ytdlpProcess) run(stdout, stderr io.Reader) {
 		p.scanErrors(stderr)
 	}()
 
-	err := p.cmd.Wait()
+	// Drain the readers to EOF before Wait: Wait closes the pipes on
+	// process exit and would discard any buffered output still unread.
 	wg.Wait()
+	err := p.cmd.Wait()
 
 	p.mu.Lock()
 	defer p.mu.Unlock()