fix: save_bom add db.session.flush() to fix unique constraint conflict

inventory-backend/app/services/bom_service.py

@@ -194,6 +194,9 @@ class BomService:
         for rec in old_records:
             db.session.delete(rec)
 
+        # 【核心修复】：强制立即执行 DELETE 语句，为后续的 INSERT 腾出唯一键空间
+        db.session.flush()
+
         for child in children:
             bom = BomTable(
                 bom_no=bom_no,