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 {