perf(S01/T02): Add MusicBrainz search/lookup/browse client, ListenBrain…

- backend/explore/types.go
- backend/explore/musicbrainz.go
- backend/explore/listenbrainz.go
- backend/explore/coverart.go
- backend/explore/coverart_test.go

backend/explore/coverart.go

@@ -0,0 +1,18 @@
+package explore
+
+import "fmt"
+
+const coverArtBaseURL = "https://coverartarchive.org/release"
+
+// CoverArtURL returns the Cover Art Archive URL for the 250px
+// front cover of the given release MBID.
+func CoverArtURL(releaseMBID string) string {
+	return fmt.Sprintf("%s/%s/front-250", coverArtBaseURL, releaseMBID)
+}
+
+// CoverArtURLSize returns the Cover Art Archive URL for the front
+// cover of the given release MBID at the specified pixel size.
+// Common sizes are 250, 500, and 1200.
+func CoverArtURLSize(releaseMBID string, size int) string {
+	return fmt.Sprintf("%s/%s/front-%d", coverArtBaseURL, releaseMBID, size)
+}