Function start_acme_background 

pub fn start_acme_background(
    domain: String,
    cert_dir: PathBuf,
    email: String,
    staging: bool,
    subdomains: Vec<String>,
)

Start background ACME provisioning and renewal. Runs initial check after a short delay (to let proxy start), then every 24h. After provisioning/renewal, hot-reloads the proxy’s TLS config automatically. If provisioning with subdomains fails, retries with bare domain only.