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/progress.go

@@ -25,8 +25,6 @@ func parseProgressLine(line string) *progressUpdate {
 	if line == "" || !strings.HasPrefix(line, "[download]") {
 		return nil
 	}
-	// Try to extract the simple percentage from the default format:
-	// [download] 45.2% of 123.45MiB at 2.34MiB/s ETA 00:45
 	m := progressRE.FindStringSubmatch(line)
 	if m == nil {
 		return nil
@@ -38,12 +36,10 @@ func parseProgressLine(line string) *progressUpdate {
 
 	u := &progressUpdate{pct: pct}
 
-	// Extract speed and ETA from remaining parts
 	rest := line
 	if idx := strings.Index(rest, "%"); idx >= 0 {
 		rest = rest[idx+1:]
 	}
-	// Look for "at <speed>"
 	if atIdx := strings.Index(rest, " at "); atIdx >= 0 {
 		afterAt := rest[atIdx+4:]
 		if spaceIdx := strings.Index(afterAt, " "); spaceIdx >= 0 {
@@ -51,7 +47,6 @@ func parseProgressLine(line string) *progressUpdate {
 			rest = afterAt[spaceIdx:]
 		}
 	}
-	// Look for "ETA <duration>"
 	if etaIdx := strings.Index(rest, "ETA "); etaIdx >= 0 {
 		etaStr := strings.TrimSpace(rest[etaIdx+4:])
 		u.eta = etaStr
@@ -63,9 +58,16 @@ func parseProgressLine(line string) *progressUpdate {
 // readProgress reads yt-dlp stderr and sends progress updates to the channel.
 // The caller is responsible for closing the updates channel.
 func readProgress(stderr io.ReadCloser, updates chan<- *domain.DownloadJob, job *domain.DownloadJob) {
+	var stderrBuf strings.Builder
 	scanner := bufio.NewScanner(stderr)
 	for scanner.Scan() {
 		line := scanner.Text()
+
+		if stderrBuf.Len() < 4096 {
+			stderrBuf.WriteString(line)
+			stderrBuf.WriteByte('\n')
+		}
+
 		parsed := parseProgressLine(line)
 		if parsed == nil {
 			continue
@@ -75,4 +77,5 @@ func readProgress(stderr io.ReadCloser, updates chan<- *domain.DownloadJob, job
 		job.ETA = parsed.eta
 		updates <- job
 	}
+	job.StderrLog = stderrBuf.String()
 }

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
 			}