How to save registered user in database with yii2?

I try to save a registerd user in the database with an api call.

So I have an model User.php:

<?php

namespace appmodels;
use Yii;
use yiibaseNotSupportedException;
use yiibehaviorsTimestampBehavior;
use yiidbActiveRecord;
use yiiwebIdentityInterface;


class User extends yiidbActiveRecord implements IdentityInterface
{

    const STATUS_DELETED = 0;
    const STATUS_INACTIVE = 9;
    const STATUS_ACTIVE = 10;
 

 
    public function behaviors()
    {
        return [
            TimestampBehavior::class,
        ];
    }

    public static function tableName()
    {
        return '{{%user}}';
    }


    public function rules()
    {
        return [
            ['status', 'default', 'value' => self::STATUS_ACTIVE],
            ['status', 'in', 'range' => [self::STATUS_ACTIVE, self::STATUS_INACTIVE, self::STATUS_DELETED]],
        ];
    }

}

and an controller UserController.php:

<?php
// phpcs:ignoreFile

namespace appmodulesv1controllers;
use Yii;
use appmodelsUser;
use yiirestActiveController;
use yiiwebResponse;

class UserController extends ActiveController
{
    public $modelClass = 'appmodelsUser';


    public function actionSignup()
    {
        $user = new User();
        $user->load(Yii::$app->getRequest()->getBodyParams(), '');
        
        if ($user->save()) {
            Yii::$app->response->statusCode = 201;
            return ['status' => 'success', 'data' => 'User registered successfully'];
        } else {
            Yii::$app->response->statusCode = 400;
            return ['status' => 'error', 'data' => $user->errors];
        }
    }

and urlManager in web.php looks:

 'urlManager' => [
            'enablePrettyUrl' => true,
            'showScriptName' => false,
            'rules' =>  ['class' => 'yiirestUrlRule', 'controller' => 'v1/user', 'POST' => 'v1/user/signup'],
            
        ]

and databse schema looks:

$this->createTable('{{%user}}', [
            'id' => $this->primaryKey(),
            'username' => $this->string()->notNull()->unique(),
            'auth_key' => $this->string(32)->notNull(),
            'verification_token'=> $this->string()->defaultValue(null),
            'password_hash' => $this->string()->notNull(),
            'password_reset_token' => $this->string()->unique(),
            'email' => $this->string()->notNull()->unique(),
            'status' => $this->smallInteger()->notNull()->defaultValue(10),
            'created_at' => $this->integer()->notNull(),
            'updated_at' => $this->integer()->notNull(),
        ]);

But if I try to call the api call in postman:

http://localhost:8080/v1/user/signup with json object:

{
    "username": "example_user",
    "password_hash": "example_password",
    "email": "[email protected]" 
}

I get this error:

 "name": "Integrity constraint violation",
    "message": "SQLSTATE[23502]: Not null violation: 7 ERROR:  null value in column "username" of relation "user" violates not-null constraintnDETAIL:  Failing row contains (3, null, null, null, null, null, 10, 1710925298, 1710925298, null).nThe SQL being executed was: INSERT INTO "user" ("status", "created_at", "updated_at") VALUES (10, 1710925298, 1710925298) RETURNING "id"",

Question: how to resolve this error?