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/db/dbq/songs.sql.go

@@ -0,0 +1,150 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+// source: songs.sql
+
+package dbq
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
+)
+
+const linkTracksAsSong = `-- name: LinkTracksAsSong :one
+
+WITH keys AS (
+    SELECT DISTINCT song_key FROM tracks WHERE id = ANY($1::uuid[])
+), target AS (
+    SELECT song_key FROM keys ORDER BY song_key LIMIT 1
+), moved AS (
+    UPDATE tracks t
+       SET song_id = (SELECT song_key FROM target)
+     WHERE t.song_key IN (SELECT song_key FROM keys)
+       AND t.song_key <> (SELECT song_key FROM target)
+    RETURNING t.id
+)
+SELECT (SELECT song_key FROM target)::uuid AS song_key,
+       (SELECT count(*) FROM moved)::bigint AS moved
+`
+
+type LinkTracksAsSongRow struct {
+	SongKey pgtype.UUID
+	Moved   int64
+}
+
+// Song links (M498 #5438): copies of one song on different releases share a
+// song_key (migration 0076).
+// Links the tracks into one song, joining any songs they already belong to.
+// The song keeps the lowest key among them, so linking the same set again
+// changes nothing. Returns that key and how many tracks moved to it.
+func (q *Queries) LinkTracksAsSong(ctx context.Context, trackIds []pgtype.UUID) (LinkTracksAsSongRow, error) {
+	row := q.db.QueryRow(ctx, linkTracksAsSong, trackIds)
+	var i LinkTracksAsSongRow
+	err := row.Scan(&i.SongKey, &i.Moved)
+	return i, err
+}
+
+const listTrackSongKeys = `-- name: ListTrackSongKeys :many
+SELECT id, song_key FROM tracks WHERE id = ANY($1::uuid[])
+`
+
+type ListTrackSongKeysRow struct {
+	ID      pgtype.UUID
+	SongKey pgtype.UUID
+}
+
+func (q *Queries) ListTrackSongKeys(ctx context.Context, ids []pgtype.UUID) ([]ListTrackSongKeysRow, error) {
+	rows, err := q.db.Query(ctx, listTrackSongKeys, ids)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListTrackSongKeysRow
+	for rows.Next() {
+		var i ListTrackSongKeysRow
+		if err := rows.Scan(&i.ID, &i.SongKey); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
+const mergeInheritSongLink = `-- name: MergeInheritSongLink :exec
+UPDATE tracks s
+   SET song_id = l.song_key
+  FROM tracks l
+ WHERE s.id = $1::uuid
+   AND l.id = $2::uuid
+   AND s.song_id IS NULL
+   AND l.song_key <> l.id
+`
+
+type MergeInheritSongLinkParams struct {
+	SurvivorID pgtype.UUID
+	LoserID    pgtype.UUID
+}
+
+// A merge keeps the removed copy's song link: when the copy being removed was
+// linked to copies on other releases and the survivor was not, the survivor
+// joins that song.
+func (q *Queries) MergeInheritSongLink(ctx context.Context, arg MergeInheritSongLinkParams) error {
+	_, err := q.db.Exec(ctx, mergeInheritSongLink, arg.SurvivorID, arg.LoserID)
+	return err
+}
+
+const shareSongLikes = `-- name: ShareSongLikes :many
+INSERT INTO general_likes (user_id, track_id, liked_at)
+SELECT l.user_id, t.id, min(l.liked_at)
+  FROM general_likes l
+  JOIN tracks lt ON lt.id = l.track_id
+  JOIN tracks t  ON t.song_key = lt.song_key
+ WHERE lt.song_key = $1::uuid
+ GROUP BY l.user_id, t.id
+ON CONFLICT (user_id, track_id) DO NOTHING
+RETURNING user_id, track_id
+`
+
+type ShareSongLikesRow struct {
+	UserID  pgtype.UUID
+	TrackID pgtype.UUID
+}
+
+// A like on any copy of the song becomes a like on every copy, dated the
+// earliest. Returns the likes added, for the sync log.
+func (q *Queries) ShareSongLikes(ctx context.Context, songKey pgtype.UUID) ([]ShareSongLikesRow, error) {
+	rows, err := q.db.Query(ctx, shareSongLikes, songKey)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ShareSongLikesRow
+	for rows.Next() {
+		var i ShareSongLikesRow
+		if err := rows.Scan(&i.UserID, &i.TrackID); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
+const unlinkDuplicateGroupSongs = `-- name: UnlinkDuplicateGroupSongs :exec
+UPDATE tracks SET song_id = NULL
+ WHERE song_id IS NOT NULL
+   AND id IN (SELECT track_id FROM duplicate_group_members WHERE group_id = $1)
+`
+
+// "Not the same song": the group's copies stop sharing a song. Likes already
+// shared stay where they are.
+func (q *Queries) UnlinkDuplicateGroupSongs(ctx context.Context, groupID pgtype.UUID) error {
+	_, err := q.db.Exec(ctx, unlinkDuplicateGroupSongs, groupID)
+	return err
+}