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/queries/songs.sql

@@ -0,0 +1,55 @@
+-- Song links (M498 #5438): copies of one song on different releases share a
+-- song_key (migration 0076).
+
+-- name: LinkTracksAsSong :one
+-- 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.
+WITH keys AS (
+    SELECT DISTINCT song_key FROM tracks WHERE id = ANY(sqlc.arg(track_ids)::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;
+
+-- name: ShareSongLikes :many
+-- 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.
+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 = sqlc.arg(song_key)::uuid
+ GROUP BY l.user_id, t.id
+ON CONFLICT (user_id, track_id) DO NOTHING
+RETURNING user_id, track_id;
+
+-- name: UnlinkDuplicateGroupSongs :exec
+-- "Not the same song": the group's copies stop sharing a song. Likes already
+-- shared stay where they are.
+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 = sqlc.arg(group_id));
+
+-- name: ListTrackSongKeys :many
+SELECT id, song_key FROM tracks WHERE id = ANY(sqlc.arg(ids)::uuid[]);
+
+-- name: MergeInheritSongLink :exec
+-- 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.
+UPDATE tracks s
+   SET song_id = l.song_key
+  FROM tracks l
+ WHERE s.id = sqlc.arg(survivor_id)::uuid
+   AND l.id = sqlc.arg(loser_id)::uuid
+   AND s.song_id IS NULL
+   AND l.song_key <> l.id;