Move engine package under subsonic, as it should only be used by the Subsonic API.master

The idea is to move reusable code from `engine` to `core`, in future refactorings

server/subsonic/engine/scrobbler.go

@@ -0,0 +1,70 @@
+package engine
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/deluan/navidrome/log"
+	"github.com/deluan/navidrome/model"
+)
+
+type Scrobbler interface {
+	Register(ctx context.Context, playerId int, trackId string, playDate time.Time) (*model.MediaFile, error)
+	NowPlaying(ctx context.Context, playerId int, playerName, trackId, username string) (*model.MediaFile, error)
+}
+
+func NewScrobbler(ds model.DataStore, npr NowPlayingRepository) Scrobbler {
+	return &scrobbler{ds: ds, npRepo: npr}
+}
+
+type scrobbler struct {
+	ds     model.DataStore
+	npRepo NowPlayingRepository
+}
+
+func (s *scrobbler) Register(ctx context.Context, playerId int, trackId string, playTime time.Time) (*model.MediaFile, error) {
+	var mf *model.MediaFile
+	var err error
+	err = s.ds.WithTx(func(tx model.DataStore) error {
+		mf, err = s.ds.MediaFile(ctx).Get(trackId)
+		if err != nil {
+			return err
+		}
+		err = s.ds.MediaFile(ctx).IncPlayCount(trackId, playTime)
+		if err != nil {
+			return err
+		}
+		err = s.ds.Album(ctx).IncPlayCount(mf.AlbumID, playTime)
+		if err != nil {
+			return err
+		}
+		err = s.ds.Artist(ctx).IncPlayCount(mf.ArtistID, playTime)
+		return err
+	})
+
+	if err != nil {
+		log.Error("Error while scrobbling", "trackId", trackId, err)
+	} else {
+		log.Info("Scrobbled", "title", mf.Title, "artist", mf.Artist, "user", userName(ctx))
+	}
+
+	return mf, err
+}
+
+// TODO Validate if NowPlaying still works after all refactorings
+func (s *scrobbler) NowPlaying(ctx context.Context, playerId int, playerName, trackId, username string) (*model.MediaFile, error) {
+	mf, err := s.ds.MediaFile(ctx).Get(trackId)
+	if err != nil {
+		return nil, err
+	}
+
+	if mf == nil {
+		return nil, fmt.Errorf(`ID "%s" not found`, trackId)
+	}
+
+	log.Info("Now Playing", "title", mf.Title, "artist", mf.Artist, "user", userName(ctx))
+
+	info := &NowPlayingInfo{TrackID: trackId, Username: username, Start: time.Now(), PlayerId: playerId, PlayerName: playerName}
+	return mf, s.npRepo.Enqueue(info)
+}