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/player/dlna/dlnaplayer.go

@@ -45,6 +45,13 @@ type DLNAPlayer struct {
 	avTransport   *avtransport.Client
 	renderControl *renderingcontrol.Client
 
+	// coverArtPathFn returns a local filesystem path to the cached cover
+	// art image for the given CoverArtID, or an error if no path is
+	// available. When set and the resolver succeeds, the path is exposed
+	// through the local proxy and emitted as upnp:albumArtURI in DIDL-Lite
+	// so the renderer can fetch it.
+	coverArtPathFn func(coverArtID string) (string, error)
+
 	state   int // stopped, playing, paused
 	seeking bool
 
@@ -88,7 +95,7 @@ type DLNAPlayer struct {
 	resetChan   chan (time.Duration)
 }
 
-func NewDLNAPlayer(device *device.MediaRenderer) (*DLNAPlayer, error) {
+func NewDLNAPlayer(device *device.MediaRenderer, coverArtPathFn func(coverArtID string) (string, error)) (*DLNAPlayer, error) {
 	retry := retryablehttp.NewClient()
 	retry.RetryMax = 3
 	retry.RetryWaitMin = 100 * time.Millisecond
@@ -114,12 +121,47 @@ func NewDLNAPlayer(device *device.MediaRenderer) (*DLNAPlayer, error) {
 	}
 
 	return &DLNAPlayer{
-		avTransport:   avt,
-		renderControl: rc,
-		resetChan:     make(chan time.Duration),
+		avTransport:    avt,
+		renderControl:  rc,
+		resetChan:      make(chan time.Duration),
+		coverArtPathFn: coverArtPathFn,
 	}, nil
 }
 
+// buildMediaItem assembles the avtransport.MediaItem for a track. It
+// populates the audio metadata fields (artist, album, track number, sample
+// rate, bit depth, channel count, size, bitrate, duration) so the
+// resulting DIDL-Lite includes the information the renderer needs to
+// display cover art, artist, album, and a stream-format readout.
+//
+// playbackURL is the proxy URL the renderer will fetch the stream from.
+func (d *DLNAPlayer) buildMediaItem(playbackURL string, meta mediaprovider.MediaItemMetadata) avtransport.MediaItem {
+	item := avtransport.MediaItem{
+		URL:         playbackURL,
+		Title:       meta.Name,
+		ContentType: meta.MIMEType,
+		Seekable:    true,
+		Duration:    meta.Duration,
+		Artist:      strings.Join(meta.Artists, ", "),
+		Album:       meta.Album,
+		TrackNumber: meta.TrackNumber,
+		Size:        meta.Size,
+		// DIDL-Lite res@bitrate is bytes/sec; meta.BitRate is kbps from
+		// the server. Convert: kbps * 1000 / 8 = bytes/sec.
+		Bitrate:         meta.BitRate * 125,
+		SampleFrequency: meta.SampleRate,
+		BitsPerSample:   meta.BitDepth,
+		NrAudioChannels: meta.ChannelCount,
+	}
+	if meta.CoverArtID != "" && d.coverArtPathFn != nil {
+		if path, err := d.coverArtPathFn(meta.CoverArtID); err == nil {
+			artKey := d.addURLToProxy(path)
+			item.AlbumArtURI = d.urlForItem(artKey)
+		}
+	}
+	return item
+}
+
 func (d *DLNAPlayer) SetVolume(vol int) error {
 	if d.destroyed {
 		return nil
@@ -153,12 +195,7 @@ func (d *DLNAPlayer) PlayFile(urlstr string, meta mediaprovider.MediaItemMetadat
 	d.metaLock.Unlock()
 	key := d.addURLToProxy(urlstr)
 
-	media := avtransport.MediaItem{
-		URL:         d.urlForItem(key),
-		Title:       meta.Name,
-		ContentType: meta.MIMEType,
-		Seekable:    true,
-	}
+	media := d.buildMediaItem(d.urlForItem(key), meta)
 
 	if err := d.playAVTransportMedia(&media); err != nil {
 		return err
@@ -223,12 +260,8 @@ func (d *DLNAPlayer) SetNextFile(url string, meta mediaprovider.MediaItemMetadat
 		d.ensureSetupProxy()
 
 		key := d.addURLToProxy(url)
-		media = &avtransport.MediaItem{
-			URL:         d.urlForItem(key),
-			ContentType: meta.MIMEType,
-			Title:       meta.Name,
-			Seekable:    true,
-		}
+		item := d.buildMediaItem(d.urlForItem(key), meta)
+		media = &item
 	} else {
 		// empty media item to signify erasing next track in device queue
 		media = &avtransport.MediaItem{}