feat(subsonic): emit coverArt id unconditionally (#296)

getCoverArt now falls back to sidecar images next to the first track in
an album, so browse responses can advertise a coverArt id whether or not
albums.cover_art_path is set. Worst case the client gets a Subsonic 70
when no sidecar exists.

internal/subsonic/types.go

@@ -288,12 +288,11 @@ func songRef(t dbq.Track, albumTitle, artistName string) SongRef {
 	return s
 }
 
-// coverArtID uses the album ID as the cover-art key since Minstrel's getCoverArt
-// will resolve from albums.cover_art_path. Returns "" when no cover art exists.
+// coverArtID returns the album UUID as the cover-art key. getCoverArt uses
+// the album row to find art either in cover_art_path (when the scanner sets
+// it) or via sidecar lookup in the album directory, so emitting the id
+// unconditionally is safe — worst case getCoverArt returns a Subsonic 70.
 func coverArtID(a dbq.Album) string {
-	if a.CoverArtPath == nil || *a.CoverArtPath == "" {
-		return ""
-	}
 	return uuidToID(a.ID)
 }