Relationship between the expression levels of serum galactose lectin 9 (GAL9), brain-derived neurotrophic factor (BDNF), and plasminogen activator inhibitor type 1 (PAI-1) of patients and their endometriosis r AFS stage and the predictive value of the levels of serum GAL 9, BDNF, and PAI-1 for postoperative recurrence of endometriosis