Find and remove duplicate songs within the slave library

A new Duplicates filter in Review finds songs filed more than once in the
slave library, such as a single next to its album version, and recommends
which copy to keep. Removed copies go to the quarantine and can be restored.

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

internal/engine/ops.go

@@ -622,7 +622,7 @@ func (e *Engine) Undo(ctx context.Context, opID int64) error {
 		return errors.New("the quarantined original was already purged")
 	}
 	switch o.Action {
-	case store.ActLink, store.ActPromote, store.ActBuild, store.ActRemove, store.ActRename, store.ActRemoveFolder:
+	case store.ActLink, store.ActPromote, store.ActBuild, store.ActRemove, store.ActRename, store.ActRemoveFolder, store.ActRemoveDupe:
 	default:
 		// History can hold operations of features that were removed (tag
 		// edits, file moves); undoing them as a link would be wrong.
@@ -643,6 +643,16 @@ func (e *Engine) Undo(ctx context.Context, opID int64) error {
 		pruneEmpty(filepath.Dir(o.QuarantinePath), l.quarantine)
 		return nil
 	}
+	if o.Action == store.ActRemoveDupe {
+		if err := e.undoRemoveDupe(ctx, l, o); err != nil {
+			return err
+		}
+		if err := e.st.MarkUndone(ctx, o.ID); err != nil {
+			return err
+		}
+		e.log.Info("duplicate restored from quarantine", "op", o.ID, "slave", o.RelSlave)
+		return nil
+	}
 	if o.Action == store.ActRename {
 		if err := undoRename(l, o); err != nil {
 			return err