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.

server/plugins/site-cache-purge.ts

@@ -1,48 +0,0 @@
-// Keeps the site cache purge settings out of the public site config.
-//
-// nuxt-site-config (pulled in by @nuxtjs/seo) copies every NUXT_SITE_* and
-// NUXT_PUBLIC_SITE_* environment variable into the site config, camel-cased,
-// and Nuxt serializes the site config into every page's payload (and into
-// window.__NUXT_SITE_CONFIG__ on client-only pages). The purge token's
-// earlier name NUXT_SITE_CACHE_PURGE_TOKEN therefore reached every page as
-// cachePurgeToken, and NUXT_SITE_CACHE_ALLOW_PROXIED as cacheAllowProxied.
-// The settings now live under NUXT_CACHE_PURGE_TOKEN and
-// NUXT_CACHE_PURGE_ALLOW_PROXIED, outside that prefix. The old names keep
-// working for one release, so this plugin also removes the two keys from
-// every entry of the site config stack on nuxt-site-config's
-// `site-config:init` hook, before anything reads it. The stripping stays
-// with the new names too: a site that still sets an old variable, or sets
-// both, must not publish it.
-//
-// When a setting comes from an old name, the plugin logs one deprecation
-// warning per process (globalThis[SITE_PURGE_WARNED], so a site extending
-// several layers that ship this file warns once). The warning names
-// variables, never a value.
-//
-// Without nuxt-site-config the hook is never called and the plugin does
-// nothing else. Shared: several layers ship this file byte for byte;
-// running it more than once is harmless.
-
-import { SITE_PURGE_WARNED, redactSiteConfigStack, resolveSitePurgeConfig, sitePurgeDeprecationMessage } from '../lib/siteCachePurge.ts'
-
-interface SiteConfigInitContext {
-  siteConfig?: { stack?: unknown }
-}
-
-type LooseHooks = { hook: (name: string, fn: (ctx: SiteConfigInitContext | undefined) => void) => unknown }
-
-export default defineNitroPlugin((nitroApp) => {
-  const g = globalThis as Record<symbol, unknown>
-  if (!g[SITE_PURGE_WARNED]) {
-    const message = sitePurgeDeprecationMessage(resolveSitePurgeConfig(useRuntimeConfig()).deprecated)
-    if (message) {
-      g[SITE_PURGE_WARNED] = true
-      console.warn(message)
-    }
-  }
-
-  // The hook name is untyped in an app without nuxt-site-config.
-  ;(nitroApp.hooks as unknown as LooseHooks).hook('site-config:init', (ctx) => {
-    redactSiteConfigStack(ctx?.siteConfig?.stack)
-  })
-})