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/playlists/system.go

@@ -1070,6 +1070,10 @@ func insertSystemPlaylist(ctx context.Context, qtx *dbq.Queries, userID pgtype.U
 		return pgtype.UUID{}, fmt.Errorf("insert playlist row: %w", err)
 	}
 
+	tracks, err = oneCopyPerSong(ctx, qtx, tracks)
+	if err != nil {
+		return pgtype.UUID{}, err
+	}
 	for _, t := range tracks {
 		var pickKind *string
 		if t.PickKind != "" {
@@ -1096,6 +1100,34 @@ func insertSystemPlaylist(ctx context.Context, qtx *dbq.Queries, userID pgtype.U
 	return p.ID, nil
 }
 
+// oneCopyPerSong keeps the first of a song's copies on different releases
+// (M498 #5438): a mix holds the song once, in its best-ranked place.
+func oneCopyPerSong(ctx context.Context, qtx *dbq.Queries, tracks []rankedCandidate) ([]rankedCandidate, error) {
+	ids := make([]pgtype.UUID, len(tracks))
+	for i, t := range tracks {
+		ids[i] = t.TrackID
+	}
+	rows, err := qtx.ListTrackSongKeys(ctx, ids)
+	if err != nil {
+		return nil, fmt.Errorf("song keys: %w", err)
+	}
+	songOf := make(map[pgtype.UUID]pgtype.UUID, len(rows))
+	for _, r := range rows {
+		songOf[r.ID] = r.SongKey
+	}
+	seen := make(map[pgtype.UUID]bool, len(tracks))
+	out := tracks[:0:0]
+	for _, t := range tracks {
+		song, ok := songOf[t.TrackID]
+		if ok && seen[song] {
+			continue
+		}
+		seen[song] = true
+		out = append(out, t)
+	}
+	return out, nil
+}
+
 // uuidStringPL renders a pgtype.UUID as the canonical 8-4-4-4-12 form.
 // (pgtype.UUID's String method exists on some pgx versions but not others;
 // also the playlists package needs this independent of test helpers.)