feat: the same song on several releases counts as one song (M498 #5438)

A single and the album it is on stay two files, since each fulfils its own
release in Lidarr, but they are one song to the listener.

- tracks.song_id links copies; the generated song_key (song_id, else the
  track's own id) is what they share (migration 0076).
- The resolver links each cross-release group every pass (idempotent; only
  with auto-resolve on) and, when a link is new, shares existing likes across
  the song and logs them for sync.
- A like or unlike (web and Subsonic) reaches every copy; each change is
  logged and published so clients update every heart.
- The Liked list and its count show the song once; Shuffle and the mix writer
  take one copy per song.
- A merge keeps the removed copy's song link; "Not the same song" on the
  Across releases tab dismisses the group and undoes the link.

Shared plays ("heard via another copy") are left for a later step.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/duplicate_resolve.go

@@ -89,6 +89,8 @@ type DuplicateResolveResult struct {
 	Merged          int
 	MergeFailed     int
 	ReleaseChanges  []ReleaseChange
+	// SongsLinked counts cross-release groups newly linked as one song.
+	SongsLinked int
 }
 
 // LidarrPathKey is the part of a path Minstrel and Lidarr agree on: the last
@@ -177,7 +179,18 @@ func ResolveDuplicates(
 		return res, err
 	}
 
-	if !act || !res.LidarrConsulted {
+	if !act {
+		return res, nil
+	}
+
+	// Linking removes nothing and needs nothing from Lidarr.
+	linked, err := linkCrossRelease(ctx, pool, logger, groups)
+	res.SongsLinked = linked
+	if err != nil {
+		return res, err
+	}
+
+	if !res.LidarrConsulted {
 		return res, nil
 	}
 
@@ -222,6 +235,66 @@ func ResolveDuplicates(
 	return res, nil
 }
 
+// linkCrossRelease links each cross-release group's copies as one song
+// (#5438): a like on one is a like on all, and the Liked list, shuffle and the
+// mixes count the song once. Linking an already-linked group changes nothing,
+// so this runs every pass; likes are shared only when a link is new, since a
+// like made after it reaches every copy as it is made.
+func linkCrossRelease(ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, groups []*resolveGroup) (int, error) {
+	linked := 0
+	for _, g := range groups {
+		if g.class != ClassCrossRelease {
+			continue
+		}
+		if ctx.Err() != nil {
+			return linked, ctx.Err()
+		}
+		ids := make([]pgtype.UUID, len(g.members))
+		for i, m := range g.members {
+			ids[i] = m.TrackID
+		}
+		moved, err := linkSong(ctx, pool, ids)
+		if err != nil {
+			logger.Warn("duplicate resolve: song link failed", "group_id", syncpkg.FormatUUID(g.id), "err", err)
+			continue
+		}
+		if moved {
+			linked++
+		}
+	}
+	return linked, nil
+}
+
+// linkSong links the tracks as one song and, when that changed anything,
+// shares their likes across the song and logs each added like for sync.
+func linkSong(ctx context.Context, pool *pgxpool.Pool, ids []pgtype.UUID) (bool, error) {
+	tx, err := pool.Begin(ctx)
+	if err != nil {
+		return false, err
+	}
+	defer func() { _ = tx.Rollback(ctx) }()
+	tq := dbq.New(tx)
+	link, err := tq.LinkTracksAsSong(ctx, ids)
+	if err != nil {
+		return false, fmt.Errorf("link: %w", err)
+	}
+	if link.Moved == 0 {
+		return false, nil
+	}
+	added, err := tq.ShareSongLikes(ctx, link.SongKey)
+	if err != nil {
+		return false, fmt.Errorf("share likes: %w", err)
+	}
+	likeIDs := make([]string, len(added))
+	for i, a := range added {
+		likeIDs[i] = syncpkg.EncodeLikeID(syncpkg.FormatUUID(a.UserID), syncpkg.FormatUUID(a.TrackID))
+	}
+	if err := syncpkg.LogChanges(ctx, tx, syncpkg.EntityLikeTrack, likeIDs, syncpkg.OpUpsert); err != nil {
+		return false, fmt.Errorf("log shared likes: %w", err)
+	}
+	return true, tx.Commit(ctx)
+}
+
 func foldResolveGroups(rows []dbq.ListDuplicateGroupsForResolveRow) []*resolveGroup {
 	var groups []*resolveGroup
 	for _, r := range rows {
@@ -250,7 +323,7 @@ func resolveNote(g *resolveGroup) string {
 			return "Lidarr tracks every copy as its own track, so removing one would make Lidarr download it again."
 		}
 	case ClassCrossRelease:
-		return "Each copy belongs to its own release, and Lidarr keeps each one. Nothing is removed."
+		return "Each copy belongs to its own release, and Lidarr keeps each one. Nothing is removed; they count as one song."
 	case ClassMismatch:
 		return "Identical audio filed under different titles: one file carries another song's tags."
 	}
@@ -674,5 +747,5 @@ func (w *DuplicateResolveWorker) tickOnce(ctx context.Context) {
 	}
 	w.logger.Info("duplicate resolve complete",
 		"groups", res.Groups, "lidarr", res.LidarrConsulted, "merged", res.Merged,
-		"merge_failed", res.MergeFailed, "release_changes", len(res.ReleaseChanges))
+		"merge_failed", res.MergeFailed, "release_changes", len(res.ReleaseChanges), "songs_linked", res.SongsLinked)
 }