feat: IndexNow key, key file and pings

- saving with IndexNow on generates a 32-hex key (crypto/rand);
  indexnow_new_key generates a new one, so the old key file answers 404
- GET /_sitemap/indexnow/{file} serves <key>.txt as text/plain while
  IndexNow is on; the SEO layer maps it to the site root
- after each rebuild in production the added, changed and removed
  addresses on the site host are queued as golem15.sitemap.indexnow jobs
  (at most 10 000 per request, five attempts)
- the job posts {host, key, urlList} to api.indexnow.org through a
  fetchguard client limited to that host and records the answer; 429 and
  5xx retry, 400, 403 and 422 do not
- the IndexNow tab shows the S2 status partial: key or no key, last
  notification or never, and the 403, 422 and 429 callouts

controllers/settings/_indexnow_status.htm

@@ -0,0 +1,15 @@
+<div class="summer-callout" role="status">
+<p class="summer-callout__text">{{ .Data.Key }}</p>
+<p class="summer-callout__text">{{ .Data.Last }}</p>
+</div>
+{{- if .Data.Callout -}}
+{{- if eq .Data.Severity "warning" -}}
+<div class="summer-callout summer-callout--warning" role="alert">
+<p class="summer-callout__text">{{ .Data.Callout }}</p>
+</div>
+{{- else -}}
+<div class="summer-callout" role="status">
+<p class="summer-callout__text">{{ .Data.Callout }}</p>
+</div>
+{{- end -}}
+{{- end -}}