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/migrations/0076_track_song_link.down.sql

@@ -0,0 +1,3 @@
+DROP INDEX IF EXISTS tracks_song_key_idx;
+ALTER TABLE tracks DROP COLUMN IF EXISTS song_key;
+ALTER TABLE tracks DROP COLUMN IF EXISTS song_id;

internal/db/migrations/0076_track_song_link.up.sql

@@ -0,0 +1,15 @@
+-- 0076_track_song_link.up.sql — the same song on several releases counts as
+-- one song (Scribe milestone #498, #5438; operator decision D-b).
+--
+-- A single and the album it came from each hold the song, and Lidarr keeps
+-- both: each fulfils its own release. Minstrel keeps both files and links
+-- them, so a like on one is a like on the song, and a mix or radio picks the
+-- song once rather than once per release.
+--
+-- song_id is set only on a linked track, to the song_key the link chose.
+-- song_key is what everything reads: the track's own id until it is linked.
+-- No foreign key: the track whose id names a song may itself be merged away
+-- later, and the key still groups the copies left.
+ALTER TABLE tracks ADD COLUMN song_id uuid;
+ALTER TABLE tracks ADD COLUMN song_key uuid GENERATED ALWAYS AS (COALESCE(song_id, id)) STORED;
+CREATE INDEX tracks_song_key_idx ON tracks (song_key);