certs: async renewal via background task + status polling

- POST /acme/renew returns a request_id and spawns a background task
  (renew/deploy/refresh steps); dedups per-domain like issue
- completes as "skipped" when acme.sh reports the renewal window
  has not been reached (no --force)
- new GET /acme/renew/status endpoint (iface + handler + blueprint)
- run acme.sh subprocesses off the event loop (asyncio.to_thread)
  in both issuance and renewal
- ActionCell: busy/busyLabel props; certs page disables the Renew
  button and polls renewal status with toasts for success/skip/fail

docs/hoover.md

@@ -1052,6 +1052,8 @@ ActionCell({
 | `removeLabel` | Delete button label (default: `'Remove'`) |
 | `removeBody` | Optional JSON body to send with DELETE |
 | `editCls` | Override classes for edit button (default: `'btn btn-sm btn-outline'`) |
+| `busy` | When `true` the action button is disabled and shows `busyLabel` (use for in-flight operations). |
+| `busyLabel` | Label shown while `busy` (default: `editLabel` + `'…'`) |
 | `deleteKey` | Unique identifier forwarded to `ConfirmDelete`. Enables pending-delete row styling. |
 
 #### `certStatusBadge(props)`