feat(user): add user modification API and handler

implement user modification functionality including:
- new endpoint /userMgr/modify
- database update function DBUpdateUser
- request/response structures
- handler with proper authentication and validation
- updated API documentation
- marked as completed in README checklist

router.go

@@ -21,6 +21,7 @@ func setupRoutes() {
 	http.HandleFunc("/userMgr/create", CreateUserHandler)
 	http.HandleFunc("/userMgr/remove", RemoveUserHandler)
 	http.HandleFunc("/userMgr/list", ListUserHandler)
+	http.HandleFunc("/userMgr/modify", ModifyUserHandler)
 	http.HandleFunc("/userMgr/modifyType", ModifyUserTypeHandler)
 
 	http.HandleFunc("/sessionMgr/list", ListActiveSessionsHandler)