feat!: extend the SEO layer and drop the copied purge route

- extend vue-seo-layer (link dependency during development); every page
  takes its head from the SEO plugin through it
- remove the copied shared purge route, library, plugin and their test:
  POST /api/__site__/cache/purge comes from vue-seo-layer with the same
  contract and environment variables
- the page-less playground sets seo.htmlSitemap false; the smoke points
  NUXT_SEO_API_BASE at the fixture API (fails open)

BREAKING CHANGE: server/api/__site__/cache/purge.post.ts,
server/lib/siteCachePurge.ts and server/plugins/site-cache-purge.ts are
no longer in this layer, and the runtime keys cachePurge and siteCache
are declared by vue-seo-layer. An app without pages sets
seo.htmlSitemap false.

nuxt.config.ts

@@ -4,7 +4,9 @@
 //
 // Every path below is absolute and built from this file's directory: a
 // relative or ~ path in a layer config resolves against the consuming
-// project, not the layer. The layer declares no Nuxt modules.
+// project, not the layer. The layer declares no Nuxt modules; it extends
+// vue-seo-layer, which owns the page head and the shared site cache purge
+// route POST /api/__site__/cache/purge.
 import { fileURLToPath } from 'node:url'
 
 const currentDir = fileURLToPath(new URL('.', import.meta.url))
@@ -13,6 +15,9 @@ export default defineNuxtConfig({
   compatibilityDate: '2025-07-15',
   devtools: { enabled: false },
 
+  // Resolved from this layer's own node_modules.
+  extends: ['vue-seo-layer'],
+
   // Tokens first, then the neutral styles that use them. The app's own css
   // array loads after these, so its token overrides win.
   css: [`${currentDir}app/assets/css/faq-tokens.css`, `${currentDir}app/assets/css/faq-base.css`],
@@ -23,31 +28,6 @@ export default defineNuxtConfig({
       // from request input.
       apiBase: 'http://127.0.0.1:8096/_faq/api/v1',
     },
-    cachePurge: {
-      // Server-only bearer token of the shared site cache purge route
-      // POST /api/__site__/cache/purge (NUXT_CACHE_PURGE_TOKEN), the
-      // backend's site_cache.purge_token. Shorter than 32 characters, the
-      // route answers 404. Not under NUXT_SITE_: nuxt-site-config copies
-      // every NUXT_SITE_* variable into the site config, which is published
-      // in every page's payload.
-      token: '',
-      // NUXT_CACHE_PURGE_ALLOW_PROXIED: set it to true when the backend
-      // reaches the site through a TLS-terminating proxy (an https
-      // site_cache.purge_url on another host), which adds X-Forwarded-For,
-      // X-Real-IP or Forwarded. The route then accepts proxied requests and
-      // the bearer token is the only gate; unset, it answers 404 to any
-      // proxied request.
-      allowProxied: false,
-    },
-    // Deprecated (one release): the earlier names of cachePurge.token
-    // (NUXT_SITE_CACHE_PURGE_TOKEN) and cachePurge.allowProxied
-    // (NUXT_SITE_CACHE_ALLOW_PROXIED), read when the new ones are unset. The
-    // layer's server/plugins/site-cache-purge.ts strips them from the site
-    // config and logs a deprecation warning.
-    siteCache: {
-      purgeToken: '',
-      allowProxied: false,
-    },
     public: {
       faq: {
         // Same-origin base for browser fetches (NUXT_PUBLIC_FAQ_API_BASE).
@@ -55,12 +35,4 @@ export default defineNuxtConfig({
       },
     },
   },
-
-  // The shared site routes are never route-cached. Nitro wraps every
-  // handler matching an app rule such as '/**': { swr } in its cached
-  // handler, which keys by path, does not skip POST and hides request
-  // headers, so the purge route would answer from the cache without running.
-  routeRules: {
-    '/api/__site__/**': { cache: false },
-  },
 })