fix: address code review findings

internal/musicbrainz/client.go

@@ -35,6 +35,12 @@ func NewClient(cfg config.MusicBrainzConfig) *MusicBrainzClient {
 	}
 }
 
+// Close releases resources held by the client, draining any idle keep-alive
+// connections so they don't linger until garbage collection.
+func (c *MusicBrainzClient) Close() {
+	c.httpClient.CloseIdleConnections()
+}
+
 // doGet performs a rate-limited HTTP GET request to the MusicBrainz API.
 // It blocks until the rate limiter allows the request, then sets the proper
 // User-Agent header and returns the response body.