feat: token-protected site cache purge route

- POST /api/__journal__/cache/purge removes every Nitro route-cache entry
- 404 without a 32-character token or behind a proxy, 401 on a wrong bearer
  token (constant-time over sha256 digests)
- routeRules keep /api/__journal__/** out of the route cache
- runtimeConfig.journal.cachePurgeToken (NUXT_JOURNAL_CACHE_PURGE_TOKEN)
- fixture API reload(); testing/purge-check.mjs proves a cached page shows
  a change after a purge; the playground caches with swr 300

server/api/__journal__/cache/purge.post.ts

@@ -0,0 +1,36 @@
+// POST /api/__journal__/cache/purge: removes every Nitro route-cache entry
+// (pages, _payload.json and feeds), so a Journal save in the admin shows on
+// the next page view. The backend calls it on loopback after the save
+// commits, with `Authorization: Bearer <runtimeConfig.journal.cachePurgeToken>`
+// and the body {"kind":"post"|"category","slug":"<slug>"}; the answer is
+// {"purged":<n>}.
+//
+// 404 without a token of at least 32 characters, and for any request that
+// came through a reverse proxy (X-Forwarded-For, X-Real-IP or Forwarded);
+// 401 for a missing or wrong bearer token. The layer's routeRules keep this
+// route out of the route cache.
+
+import { authorizePurge, parsePurgeEvent, purgeRouteCache } from '../../../../app/utils/siteCache.ts'
+
+const PROXY_HEADERS = ['x-forwarded-for', 'x-real-ip', 'forwarded']
+
+export default defineEventHandler(async (event) => {
+  setResponseHeader(event, 'cache-control', 'no-store')
+  const config = useRuntimeConfig(event)
+  const verdict = authorizePurge({
+    token: (config.journal as { cachePurgeToken?: unknown }).cachePurgeToken,
+    authorization: getRequestHeader(event, 'authorization'),
+    proxied: PROXY_HEADERS.some((name) => getRequestHeader(event, name) !== undefined),
+  })
+  if (verdict === 'disabled' || verdict === 'proxied') {
+    throw createError({ statusCode: 404, statusMessage: 'Not Found' })
+  }
+  if (verdict === 'unauthorized') {
+    throw createError({ statusCode: 401, statusMessage: 'Unauthorized' })
+  }
+
+  const { kind, slug } = parsePurgeEvent(await readBody(event).catch(() => null))
+  const purged = await purgeRouteCache(useStorage('cache'))
+  console.info(`[journal] cache purge: removed ${purged} route cache entries (${kind} ${slug})`)
+  return { purged }
+})