feat(19.3-04): shared site cache purge route POST /api/__site__/cache/purge

- server/lib/siteCachePurge.ts: authorizeSitePurge, purgeSiteRouteCache,
  parseSitePurgeEvent (kinds: lower-case letters and underscores)
- the route reads runtimeConfig.siteCache.purgeToken
  (NUXT_SITE_CACHE_PURGE_TOKEN), 404 when off or proxied, 401 unauthorized
- route rule '/api/__site__/**': { cache: false }
- sitePurge specs with sha256 pins of the two shared files
- version 0.2.0

package.json

@@ -1,6 +1,6 @@
 {
   "name": "vue-faq-layer",
-  "version": "0.1.0",
+  "version": "0.2.0",
   "private": true,
   "type": "module",
   "main": "./nuxt.config.ts",