feat(subsonic): return artist cover ids for similar artists response

server/ctrlsubsonic/handlers_by_tags.go

@@ -367,11 +367,13 @@ func (c *Controller) ServeGetArtistInfoTwo(r *http.Request) *spec.Response {
 		}
 		artistID := &specid.ID{}
 		if artist.ID != 0 {
+			// we don't always have a match if `inclNotPresent`
 			artistID = artist.SID()
 		}
 		sub.ArtistInfoTwo.SimilarArtist = append(sub.ArtistInfoTwo.SimilarArtist, &spec.SimilarArtist{
 			ID:         artistID,
 			Name:       similarInfo.Name,
+			CoverArt:   artistID,
 			AlbumCount: artist.AlbumCount,
 		})
 	}

server/ctrlsubsonic/spec/spec.go

@@ -287,6 +287,7 @@ type Playlist struct {
 type SimilarArtist struct {
 	ID         *specid.ID `xml:"id,attr"                   json:"id"`
 	Name       string     `xml:"name,attr"                 json:"name"`
+	CoverArt   *specid.ID `xml:"coverArt,attr"             json:"coverArt"`
 	AlbumCount int        `xml:"albumCount,attr,omitempty" json:"albumCount,omitempty"`
 }