Fix GetTrackDownloadInfo to return signed download URLs

- Pass quality parameter to /tracks/{track_id}/download-info API request
- Add GetTrackDownloadURL method that resolves signed download URLs by
  downloading XML from download_info_url, parsing host/path/ts/s,
  computing MD5 signature (salt: XGRlBW9FXlekgbPrRHuSiA), and
  returning https://{host}/get-mp3/{sign}/{ts}{path}
- Add DownloadInfoURL field to DownloadInfo model for the XML URL
- Update DownloadTrack to use GetDirectLink resolution when URL is empty
- Port signing logic from yandex-music-api-python download_info.py

internal/models/track.go

@@ -165,10 +165,12 @@ type TrackLyricsInfo struct {
 
 // DownloadInfo represents file download info for a track.
 type DownloadInfo struct {
-	Quality    string `json:"quality"`
-	Codec      string `json:"codec"`
-	Bitrate    int    `json:"bitrate"`
-	Size       int64  `json:"size"`
-	URL        string `json:"url"`
-	DirectLink bool   `json:"direct_link"`
+	Quality         string `json:"quality"`
+	Codec           string `json:"codec"`
+	Bitrate         int    `json:"bitrate"`
+	Size            int64  `json:"size"`
+	URL             string `json:"url"`
+	DirectLink      bool   `json:"direct_link"`
+	DownloadInfoURL string `json:"download_info_url"`
+	DirectLinkURL   string `json:"-"` // Computed signed download URL
 }