Endpoint for organization admins to generate an access token scoped to their organization.
Args: org_name (str): The name (or ID) of the organization for which the token is being generated. expiry_duration (Optional[int]): Token expiry duration in seconds. If not provided, uses default expiry.
Returns:
dict: A dictionary containing the org_token.
Raises: HTTPException 403: If the current user is not authorized to manage the specified organization.