docs(whp): add backend-switching how-to + Optimized Webserver (OLS) add-on page

- how-to/switching-site-backend: change a site's container type between standard
  PHP/FPM and the premium LiteSpeed/OLS backend; required steps to move off
  premium before cancelling the add-on (panel blocks the disable otherwise).
- add-ons/optimized-webserver: what the OLS/LSCache add-on is, advantages, when
  NOT to use it, how to enable/use, and cancellation requirements.
- add-ons/overview: list the new add-on.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/content/docs/whp/how-to/switching-site-backend.mdx

@@ -0,0 +1,100 @@
+---
+title: Switching your site's backend
+description: Change the web engine (container type) running a site — standard PHP/FPM or the premium LiteSpeed/OpenLiteSpeed tier.
+sidebar:
+  order: 5
+---
+
+import { Steps, Aside } from '@astrojs/starlight/components';
+import SignIn from '~/content/partials/signing-in.mdx';
+import Support from '~/content/partials/support-link.mdx';
+
+Every WHP site runs inside a container. The **container type** determines the web engine that serves your site's files and runs your PHP code. Most sites use a standard PHP or PHP-FPM container, which handles the vast majority of WordPress and PHP workloads well. If you've enabled the **Optimized Webserver** add-on, you also have access to LiteSpeed/OpenLiteSpeed (OLS) container types — a premium engine known for its built-in full-page cache (LSCache) and lower memory usage under traffic.
+
+Switching backends is a one-step change in the Sites editor, but it does briefly restart your container, so plan for a few seconds of downtime.
+
+## Before you start
+
+- You need **admin access** to your WHP account.
+- If you want to switch **onto** a LiteSpeed backend, your account must have the [Optimized Webserver add-on](/whp/add-ons/optimized-webserver/) enabled. If it isn't, those container types won't appear in the dropdown.
+- About 2 minutes, plus a brief site restart.
+
+## Sign in to WHP
+
+<SignIn />
+
+## Steps
+
+<Steps>
+
+1. In the sidebar, click **Sites**.
+
+2. Find the site you want to change and click **Edit** (the pencil icon next to it).
+
+3. In the **Edit Site** form, locate the **Container Type** (or **PHP Version / Backend**) dropdown.
+
+4. Select the backend you want:
+   - **PHP** or **PHP-FPM** options — standard shared webserver tier, suitable for most WordPress and PHP sites.
+   - **LiteSpeed PHP** options (e.g. *LiteSpeed PHP 8.x*) — premium OLS tier. These only appear if the Optimized Webserver add-on is active on your account.
+
+5. Click **Save**. WHP recreates the container with the new engine. Expect a brief moment of downtime (typically a few seconds) while the container restarts.
+
+</Steps>
+
+<Aside type="note">
+  After switching backends, any server-side page cache starts cold. For LiteSpeed sites this means LSCache has nothing cached yet — your first few page loads will be slightly slower while the cache warms up. This is normal and resolves on its own within minutes.
+</Aside>
+
+## Switching onto the premium LiteSpeed backend
+
+The LiteSpeed/OpenLiteSpeed container types are part of the **Optimized Webserver** add-on. If you open the Container Type dropdown and don't see any LiteSpeed options, the add-on isn't enabled on your account yet.
+
+To enable it:
+
+1. Visit the [Optimized Webserver add-on page](/whp/add-ons/optimized-webserver/) for details and pricing.
+2. Enable it from your [client portal](https://secure.anhonesthost.com/clientarea.php).
+3. Once active, the LiteSpeed PHP options will appear in the Container Type dropdown when editing any site.
+
+## Before cancelling the Optimized Webserver add-on
+
+<Aside type="caution">
+  You must move **all** of your sites off the LiteSpeed backend before you can cancel or disable the Optimized Webserver add-on. The system will block the cancellation and show an error listing every site that still needs to be moved.
+</Aside>
+
+To prepare for cancellation, switch each affected site back to a standard PHP or PHP-FPM container type:
+
+<Steps>
+
+1. In the sidebar, click **Sites**.
+
+2. For each site currently on a LiteSpeed container type, click **Edit**.
+
+3. Change the **Container Type** to a standard **PHP** or **PHP-FPM** option.
+
+4. Click **Save** and wait for the container to restart before moving to the next site.
+
+5. Repeat until no sites are on a LiteSpeed backend.
+
+</Steps>
+
+Once all sites are on standard backends, you can cancel or disable the add-on from your [client portal](https://secure.anhonesthost.com/clientarea.php) without errors.
+
+## Troubleshooting
+
+**LiteSpeed options don't appear in the dropdown.** The Optimized Webserver add-on is not enabled on your account. See [Optimized Webserver](/whp/add-ons/optimized-webserver/) to add it.
+
+**Site doesn't come back up after switching.** Wait 30 seconds and reload — the container may still be starting. If the site is still down after a minute, contact support.
+
+**Cancelling the add-on shows an error listing sites.** Switch each listed site to a standard PHP/PHP-FPM backend first (see the steps above), then retry the cancellation.
+
+**LiteSpeed cache not serving cached pages after the switch.** This is expected — the cache starts empty after every container recreation. It warms up automatically as visitors load pages.
+
+## Related
+
+- [Create a site](/whp/how-to/create-a-site/)
+- [Optimized Webserver add-on](/whp/add-ons/optimized-webserver/)
+- [Resource upgrades](/whp/add-ons/resource-upgrades/)
+
+## Still stuck?
+
+<Support />