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

README.md

@@ -10,11 +10,12 @@ Telegram bot for downloading media from any site yt-dlp supports.
 - Thumbnail preview with metadata (title, uploader, duration) before download
 - Audio-only sites (Spotify, SoundCloud, YouTube Music, etc.) auto-detect and skip to audio quality selection
 - Multi-step format selection: type picker (Video/Audio/Ask), quality, secondary format (video+audio combine), language
-- Auto-select: skip the format picker entirely when defaults are set in user preferences
+- Auto-select: skip all pickers when both media type AND quality are set in preferences (quality "Ask" shows a filtered picker)
+- Audio remuxed to m4a (Telegram-friendly), video remuxed to mp4
 - Download progress updates (with speed and ETA)
 - Cancel downloads at any time (terminates yt-dlp process)
 - Per-user quota system (daily, weekly, monthly; 0 = unlimited)
-- User preferences (default media type, audio/video quality, language)
+- User preferences (default media type, audio/video quality with "Ask" option, language)
 - Download history with pagination
 - Configurable file size limit (default 50 MB)
 - Cookies support for age-restricted and authenticated content