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.go

@@ -13,11 +13,11 @@ import (
 // 498's body) splits into four shapes:
 //
 //   - same_release: every copy is on one album. An album holds a song once, so
-//     the extra copies are the ones to fold away — when Lidarr does not need
-//     them (the resolver checks).
+//     the extra copies are the ones to fold away.
 //   - cross_release: the copies are on different albums under one title. A
 //     single and the album it came from, a deluxe edition and the standard one:
-//     each copy fulfils its own release in Lidarr, so none is deleted.
+//     each copy belongs to its own release, so none is deleted (unless it is a
+//     video rip; the resolver decides).
 //   - mismatch: byte-identical audio on different albums under different
 //     titles. One file carries another song's tags — a wrong-file import.
 //   - review: anything else. The operator decides.
@@ -123,13 +123,6 @@ func MatchTitleKey(title, artist string) string {
 	return alnumKey(title)
 }
 
-// ReleaseTitleKey is the lighter comparison for asking whether one release
-// lists a song twice: case, "&" and punctuation only. Every word stays, so a
-// box set's "Song" and "Song (demo)" are two songs, as the release means them.
-func ReleaseTitleKey(title string) string {
-	return alnumKey(title)
-}
-
 // alnumKey lowercases, reads "&" as "and", and keeps only letters and digits.
 func alnumKey(s string) string {
 	s = strings.ReplaceAll(strings.ToLower(s), "&", " and ")