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/api/admin_duplicates.go

@@ -331,15 +331,17 @@ func (h *handlers) handleRunDuplicateSweep(w http.ResponseWriter, _ *http.Reques
 
 // handleDismissDuplicateGroup implements POST
 // /api/admin/library/duplicates/{id}/dismiss: "these are not duplicates". The
-// sweep keeps the dismissal and will not propose that set of tracks again. 404
-// duplicate_group_not_pending when the group was already resolved or is gone.
+// sweep keeps the dismissal and will not propose that set of tracks again. On a
+// cross-release group it also undoes the song link (M498 #5438): the copies
+// stop sharing likes from here on. 404 duplicate_group_not_pending when the
+// group was already resolved or is gone.
 func (h *handlers) handleDismissDuplicateGroup(w http.ResponseWriter, r *http.Request) {
 	id, ok := parseUUID(chi.URLParam(r, "id"))
 	if !ok {
 		writeAdminJSONErr(w, http.StatusBadRequest, "invalid_id")
 		return
 	}
-	n, err := dbq.New(h.pool).DismissDuplicateGroup(r.Context(), id)
+	n, err := h.dismissDuplicateGroup(r.Context(), id)
 	if err != nil {
 		h.logger.Error("admin: dismiss duplicate group", "err", err)
 		writeAdminJSONErr(w, http.StatusInternalServerError, "server_error")
@@ -352,6 +354,25 @@ func (h *handlers) handleDismissDuplicateGroup(w http.ResponseWriter, r *http.Re
 	writeJSON(w, http.StatusOK, map[string]string{"status": "dismissed"})
 }
 
+// dismissDuplicateGroup dismisses the group and unlinks its copies as one song,
+// together.
+func (h *handlers) dismissDuplicateGroup(ctx context.Context, id pgtype.UUID) (int64, error) {
+	tx, err := h.pool.Begin(ctx)
+	if err != nil {
+		return 0, err
+	}
+	defer func() { _ = tx.Rollback(ctx) }()
+	tq := dbq.New(tx)
+	n, err := tq.DismissDuplicateGroup(ctx, id)
+	if err != nil || n == 0 {
+		return n, err
+	}
+	if err := tq.UnlinkDuplicateGroupSongs(ctx, id); err != nil {
+		return 0, err
+	}
+	return n, tx.Commit(ctx)
+}
+
 // mergeDuplicateRequest chooses the copy to keep. An empty survivor_track_id
 // keeps the report's proposal.
 type mergeDuplicateRequest struct {

internal/api/likes.go

@@ -3,6 +3,7 @@ package api
 import (
 	"errors"
 	"net/http"
+	"slices"
 
 	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgtype"
@@ -50,20 +51,36 @@ func (h *handlers) handleLikeTrack(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, apierror.InternalMsg("lookup failed", err))
 		return
 	}
-	rows, err := q.LikeTrack(r.Context(), dbq.LikeTrackParams{UserID: user.ID, TrackID: id})
+	liked, err := q.LikeTrack(r.Context(), dbq.LikeTrackParams{UserID: user.ID, TrackID: id})
 	if err != nil {
 		h.logger.Error("api: like track insert", "err", err)
 		writeErr(w, apierror.InternalMsg("insert failed", err))
 		return
 	}
-	if rows == 1 {
+	if slices.Contains(liked, id) {
 		_ = playevents.CaptureContextualLikeIfPlaying(r.Context(), q, user.ID, id, h.logger)
 	}
-	h.logLikeChange(r, syncpkg.EntityLikeTrack, user.ID, id, syncpkg.OpUpsert)
-	h.publishLikeEvent(user.ID, id, "track", true)
+	// The like is on the song (M498 #5438): every copy it reached changes on
+	// the client too.
+	for _, t := range withTrack(id, liked) {
+		h.logLikeChange(r, syncpkg.EntityLikeTrack, user.ID, t, syncpkg.OpUpsert)
+		h.publishLikeEvent(user.ID, t, "track", true)
+	}
 	w.WriteHeader(http.StatusNoContent)
 }
 
+// withTrack is the tracks a like or unlike changed, led by the one asked for,
+// which is reported even when it was already in that state.
+func withTrack(id pgtype.UUID, changed []pgtype.UUID) []pgtype.UUID {
+	out := []pgtype.UUID{id}
+	for _, t := range changed {
+		if t != id {
+			out = append(out, t)
+		}
+	}
+	return out
+}
+
 func (h *handlers) handleUnlikeTrack(w http.ResponseWriter, r *http.Request) {
 	user, ok := requireUser(w, r)
 	if !ok {
@@ -74,7 +91,8 @@ func (h *handlers) handleUnlikeTrack(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	q := dbq.New(h.pool)
-	if err := q.UnlikeTrack(r.Context(), dbq.UnlikeTrackParams{UserID: user.ID, TrackID: id}); err != nil {
+	unliked, err := q.UnlikeTrack(r.Context(), dbq.UnlikeTrackParams{UserID: user.ID, TrackID: id})
+	if err != nil {
 		h.logger.Error("api: unlike track", "err", err)
 		writeErr(w, apierror.InternalMsg("delete failed", err))
 		return
@@ -83,8 +101,10 @@ func (h *handlers) handleUnlikeTrack(w http.ResponseWriter, r *http.Request) {
 		h.logger.Error("api: soft-delete contextual_likes", "err", err)
 		// Don't fail the response — soft-delete is best-effort.
 	}
-	h.logLikeChange(r, syncpkg.EntityLikeTrack, user.ID, id, syncpkg.OpDelete)
-	h.publishLikeEvent(user.ID, id, "track", false)
+	for _, t := range withTrack(id, unliked) {
+		h.logLikeChange(r, syncpkg.EntityLikeTrack, user.ID, t, syncpkg.OpDelete)
+		h.publishLikeEvent(user.ID, t, "track", false)
+	}
 	w.WriteHeader(http.StatusNoContent)
 }