Add is_active_paid_subscription() helper that checks if subscription is
non-trial, active, and not expired. Use it across all disable_remnawave_user
call sites to prevent disabling VPN access for users with paid subscriptions.
Protected paths: block_user, delete_user_account, broadcast cleanup,
channel unsubscribe, admin deactivation, webapi endpoints, cabinet
reset-trial, reset-subscription, and disable-user endpoints.
1b6bbc7131