# Update Username

Update the authenticated user's username.

**Endpoint**: `PUT /api/auth/username`
**Auth**: Required
**Content-Type**: `application/json`

## Request Body

| Field | Type | Required | Description |
|--------|------|-----------|-------------|
| username | string | Yes | New username (min 3 chars, alphanumeric and underscore only) |

### Example Request

```json
{
  "username": "new_username"
}
```

## Response (200 OK)

```json
{
  "message": "Username updated successfully"
}
```

## Error Responses

| Code | Description |
|------|-------------|
| 400 | Invalid username format |
| 401 | Invalid or expired token |
| 409 | Username already taken by another user |
