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_merge.go

@@ -25,16 +25,6 @@ var ErrDuplicateGroupNotPending = errors.New("library: duplicate group is not pe
 // ErrSurvivorNotInGroup means the copy chosen to keep is not a member of the group.
 var ErrSurvivorNotInGroup = errors.New("library: survivor is not a member of the group")
 
-// ErrCopyTrackedByLidarr means a copy the merge would remove is one Lidarr maps
-// to a track of the release it monitors (M498). Removing it would open a hole
-// that Lidarr fills by downloading the song again, so the merge refuses and
-// changes nothing.
-var ErrCopyTrackedByLidarr = errors.New("library: a copy to remove is tracked by Lidarr")
-
-// RemovableFunc reports whether the file at filePath may be removed. A merge
-// asks it about every copy it would remove, before touching anything.
-type RemovableFunc func(filePath string) bool
-
 // MergedCopy is one copy a merge kept or removed.
 type MergedCopy struct {
 	TrackID   pgtype.UUID
@@ -85,17 +75,6 @@ type MergeResult struct {
 func MergeDuplicateGroup(
 	ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, dataDir string,
 	groupID, survivorID pgtype.UUID,
-) (MergeResult, error) {
-	return MergeDuplicateGroupGuarded(ctx, pool, logger, dataDir, groupID, survivorID, nil)
-}
-
-// MergeDuplicateGroupGuarded is MergeDuplicateGroup with a check on what it may
-// remove: when removable says no to any copy that would go, the merge returns
-// ErrCopyTrackedByLidarr (wrapped with the path) and changes nothing. A nil
-// removable allows every copy.
-func MergeDuplicateGroupGuarded(
-	ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, dataDir string,
-	groupID, survivorID pgtype.UUID, removable RemovableFunc,
 ) (MergeResult, error) {
 	if logger == nil {
 		logger = slog.Default()
@@ -130,13 +109,6 @@ func MergeDuplicateGroupGuarded(
 		return MergeResult{}, err
 	}
 
-	if removable != nil {
-		for _, l := range losers {
-			if !removable(l.FilePath) {
-				return MergeResult{}, fmt.Errorf("%w: %s", ErrCopyTrackedByLidarr, l.FilePath)
-			}
-		}
-	}
 	for _, l := range losers {
 		if err := removeTrackFileOnDisk(l.FilePath); err != nil {
 			return MergeResult{}, err