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.

package.json

@@ -14,6 +14,9 @@
   "engines": {
     "node": ">=22.18"
   },
+  "dependencies": {
+    "vue-seo-layer": "link:../vue-seo-layer"
+  },
   "peerDependencies": {
     "nuxt": "4.4.8"
   },