Skip songs with less than 31 seconds, as per Last.fm specification

See https://www.last.fm/api/scrobbling#when-is-a-scrobble-a-scrobble

core/agents/lastfm/agent.go

@@ -186,6 +186,10 @@ func (l *lastfmAgent) Scrobble(ctx context.Context, userId string, scrobbles []s
 
 	// TODO Implement batch scrobbling
 	for _, s := range scrobbles {
+		if s.Duration <= 30 {
+			log.Debug(ctx, "Skipping Last.fm scrobble for short song", "track", s.Title, "duration", s.Duration)
+			continue
+		}
 		err = l.client.Scrobble(ctx, sk, ScrobbleInfo{
 			artist:      s.Artist,
 			track:       s.Title,

core/agents/lastfm/agent_test.go

@@ -293,6 +293,17 @@ var _ = Describe("lastfmAgent", func() {
 				Expect(sentParams.Get("mbid")).To(Equal(track.MbzTrackID))
 				Expect(sentParams.Get("timestamp")).To(Equal(strconv.FormatInt(ts.Unix(), 10)))
 			})
+
+			It("skips songs with less than 31 seconds", func() {
+				track.Duration = 29
+				scrobbles := []scrobbler.Scrobble{{MediaFile: *track, TimeStamp: time.Now()}}
+				httpClient.Res = http.Response{Body: ioutil.NopCloser(bytes.NewBufferString("{}")), StatusCode: 200}
+
+				err := agent.Scrobble(ctx, "user-1", scrobbles)
+
+				Expect(err).ToNot(HaveOccurred())
+				Expect(httpClient.SavedRequest).To(BeNil())
+			})
 		})
 	})