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/app.go

@@ -160,6 +160,13 @@ func StartupApp(appName, displayAppName, appVersion, appVersionTag, latestReleas
 		a.AudioCache = ac
 	}
 	a.PlaybackManager = NewPlaybackManager(a.bgrndCtx, a.ServerManager, a.AudioCache, a.LocalPlayer, &a.Config.Playback, &a.Config.Scrobbling, &a.Config.Transcoding, &a.Config.Application)
+	a.PlaybackManager.CoverArtPathFn = func(coverArtID string) (string, error) {
+		// Ensure the thumbnail is cached on disk, then return its path so
+		// the DLNA player can expose it through the local proxy as
+		// upnp:albumArtURI.
+		a.ImageManager.GetCoverThumbnail(coverArtID)
+		return a.ImageManager.GetCoverArtPath(coverArtID)
+	}
 	a.Config.Application.MaxImageCacheSizeMB = clamp(a.Config.Application.MaxImageCacheSizeMB, 1, 500)
 	a.ImageManager.SetMaxOnDiskCacheSizeBytes(int64(a.Config.Application.MaxImageCacheSizeMB) * 1_048_576)
 	a.ServerManager.SetPrefetchAlbumCoverCallback(func(coverID string) {