feat: duplicates resolve themselves without Lidarr (M502 #5517)

Lidarr is off, and with it the resolver had stopped merging: it only
merged when Lidarr had said which copies it mapped. Minstrel is now the
only program writing the library, so the rule is about the copies:

- A song held twice on one album: the extra copies merge into the one
  whose tags fit best (then lossless, larger, older).
- Across releases or under another title: when every copy but one is a
  video rip (held back, or named like one and not marked fine), the rips
  merge into the one that is not.
- Clean copies on different releases are linked as one song, as before.
- Anything else waits for the operator.

Gone with Lidarr: release switches and their settle window, the
unmapped-file read, the rip search, the merge guard and its 409/503,
lidarr.Client's release methods, and duplicate_group_members.lidarr_state
(migration 0078). The duplicates page drops the per-copy Lidarr badge
and the blocked-merge notice; past release changes stay in its history.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/duplicate_classify_test.go

@@ -59,13 +59,3 @@ func TestMatchTitleKey(t *testing.T) {
 		}
 	}
 }
-
-// The release check keeps every word: a box set's demo is its own song.
-func TestReleaseTitleKey(t *testing.T) {
-	if ReleaseTitleKey("Song (demo)") == ReleaseTitleKey("Song") {
-		t.Error("ReleaseTitleKey collapsed a demo into the song")
-	}
-	if ReleaseTitleKey("Hallelujah Money") != ReleaseTitleKey("hallelujah money!") {
-		t.Error("ReleaseTitleKey kept case or punctuation")
-	}
-}