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>

web/src/routes/admin/duplicates/duplicates.test.ts

@@ -323,6 +323,15 @@ describe('admin duplicates', () => {
       expect(createDuplicatesQuery).toHaveBeenLastCalledWith('cross_release');
     });
 
+    // Copies across releases are linked as one song; dismissing one says they
+    // are not, which is what the button names.
+    test('Across releases dismisses as Not the same song', async () => {
+      renderWith(response());
+      await fireEvent.click(screen.getByRole('tab', { name: /across releases/i }));
+      await fireEvent.click(screen.getByRole('button', { name: 'Not the same song' }));
+      expect(dismissDuplicateGroup).toHaveBeenCalledWith('g-1');
+    });
+
     test('Resolved automatically lists merges and release changes', async () => {
       renderWith(response(), [
         {