lib/model: Move String method to folder (#4964)

lib/model/folder.go

@@ -9,6 +9,7 @@ package model
 import (
 	"context"
 	"errors"
+	"fmt"
 	"math/rand"
 	"time"
 
@@ -389,3 +390,7 @@ func (f *folder) basePause() time.Duration {
 	}
 	return time.Duration(f.PullerPauseS) * time.Second
 }
+
+func (f *folder) String() string {
+	return fmt.Sprintf("%s/%s@%p", f.Type, f.folderID, f)
+}