feat: add 'Ask' quality option, language auto-select, and m4a audio remux

- Add 'ask' option to audio/video quality settings (stored as '' in DB)
- Auto-select when both media type AND quality are set; show filtered
  format picker when quality is Ask
- Auto-skip language picker when user has a matching language preference
- Remux audio-only downloads to m4a instead of webm for Telegram compat
- Update README to document the new flow and m4a audio output

internal/dl/ytdlp.go

@@ -250,8 +250,10 @@ func (c *Client) StartDownload(job *domain.DownloadJob, downloadDir string) (<-c
 	outputPath := filepath.Join(downloadDir, job.ID+"_%(id)s.%(ext)s")
 	args = append(args, "--output", outputPath)
 
-	// Remux to mp4 for video output — Telegram-friendly container.
-	if job.MediaType != domain.MediaTypeAudio {
+	// Remux to Telegram-friendly containers: mp4 for video, m4a for audio-only.
+	if job.MediaType == domain.MediaTypeAudio {
+		args = append(args, "--merge-output-format", "m4a")
+	} else {
 		args = append(args, "--merge-output-format", "mp4")
 	}