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.

testing/smoke-ssr.mjs

@@ -144,7 +144,13 @@ async function main() {
   let nuxt = null
   const failures = /** @type {string[]} */ ([])
   try {
-    nuxt = await startNuxtServer({ outputDir: args.output, env: { [API_ENV]: `${api.url}${API_PREFIX}`, ...args.env } })
+    // vue-seo-layer resolves every page through the SEO API; the fixture API
+    // has none and answers 404, so the layer renders without a CMS answer
+    // (fail open) and never reaches a backend on the default port.
+    nuxt = await startNuxtServer({
+      outputDir: args.output,
+      env: { [API_ENV]: `${api.url}${API_PREFIX}`, NUXT_SEO_API_BASE: `${api.url}/_seo/api/v1`, ...args.env },
+    })
     for (const line of lines) {
       let check
       try {