Populate richer DIDL-Lite metadata for DLNA cast (#934)

Extend MediaItemMetadata with TrackNumber, Size, BitRate, SampleRate,
BitDepth, and ChannelCount (all already present on Track) and populate
them in Track.Metadata(). Thread them through dlnaplayer.go into the
avtransport.MediaItem so the renderer's DIDL-Lite includes upnp:artist,
upnp:album, upnp:originalTrackNumber, plus <res> attributes for
duration, size, bitrate, sampleFrequency, bitsPerSample, and
nrAudioChannels. The previously dropped Duration field is now
populated too.

For cover art, add a CoverArtPathFn callback on PlaybackManager wired
to ImageManager.GetCoverArtPath in app.go. The DLNA player invokes it
when a track is queued, registers the cached cover-thumbnail path
with the local proxy, and emits the resulting proxy URL as
upnp:albumArtURI so the renderer can display album art alongside the
audio.

Bumps go-upnpcast to pull in the MediaItem fields and DIDL-Lite
emission added in supersonic-app/go-upnpcast#4.

backend/mediaprovider/model.go

@@ -224,16 +224,22 @@ const (
 )
 
 type MediaItemMetadata struct {
-	Type       MediaItemType
-	MIMEType   string
-	ID         string
-	Name       string
-	Artists    []string
-	ArtistIDs  []string
-	Album      string
-	AlbumID    string
-	CoverArtID string
-	Duration   time.Duration
+	Type         MediaItemType
+	MIMEType     string
+	ID           string
+	Name         string
+	Artists      []string
+	ArtistIDs    []string
+	Album        string
+	AlbumID      string
+	CoverArtID   string
+	Duration     time.Duration
+	TrackNumber  int
+	Size         int64
+	BitRate      int // kbps as reported by the server
+	SampleRate   int // Hz
+	BitDepth     int
+	ChannelCount int
 }
 
 type MediaItem interface {
@@ -246,16 +252,22 @@ func (t *Track) Metadata() MediaItemMetadata {
 		return MediaItemMetadata{}
 	}
 	return MediaItemMetadata{
-		Type:       MediaItemTypeTrack,
-		MIMEType:   t.ContentType,
-		ID:         t.ID,
-		Name:       t.Title,
-		Artists:    t.ArtistNames,
-		ArtistIDs:  t.ArtistIDs,
-		Album:      t.Album,
-		AlbumID:    t.AlbumID,
-		CoverArtID: t.CoverArtID,
-		Duration:   t.Duration,
+		Type:         MediaItemTypeTrack,
+		MIMEType:     t.ContentType,
+		ID:           t.ID,
+		Name:         t.Title,
+		Artists:      t.ArtistNames,
+		ArtistIDs:    t.ArtistIDs,
+		Album:        t.Album,
+		AlbumID:      t.AlbumID,
+		CoverArtID:   t.CoverArtID,
+		Duration:     t.Duration,
+		TrackNumber:  t.TrackNumber,
+		Size:         t.Size,
+		BitRate:      t.BitRate,
+		SampleRate:   t.SampleRate,
+		BitDepth:     t.BitDepth,
+		ChannelCount: t.Channels,
 	}
 }