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

models/settings.go

@@ -92,7 +92,7 @@ func (Settings) TableName() string { return "golem15_sitemap_settings" }
 
 // Fillable lists the columns the settings form edits.
 func (Settings) Fillable() []string {
-	return []string{"site_url", "file_size", "extra_urls", "images_enabled", "news_enabled", "news_publication", "news_sources"}
+	return []string{"site_url", "file_size", "extra_urls", "images_enabled", "news_enabled", "news_publication", "news_sources", "indexnow_enabled"}
 }
 
 // Rules is empty: the settings controller runs Validate before a save.

models/settings/fields.yaml

@@ -99,3 +99,31 @@ fields:
             action: hide
             field: news_enabled
             condition: unchecked
+
+    indexnow_enabled:
+        label: golem15.sitemap::lang.settings.indexnow_enabled
+        comment: golem15.sitemap::lang.settings.indexnow_enabled_comment
+        type: switch
+        default: false
+        span: full
+        tab: golem15.sitemap::lang.settings.tab_indexnow
+
+    _indexnow_status:
+        type: partial
+        path: indexnow_status
+        span: full
+        tab: golem15.sitemap::lang.settings.tab_indexnow
+
+    # No column: the controller lists it in FormVirtualFields and generates
+    # a new key on save when it is checked.
+    indexnow_new_key:
+        label: golem15.sitemap::lang.settings.indexnow_new_key
+        comment: golem15.sitemap::lang.settings.indexnow_new_key_comment
+        type: checkbox
+        default: false
+        span: full
+        tab: golem15.sitemap::lang.settings.tab_indexnow
+        trigger:
+            action: hide
+            field: indexnow_enabled
+            condition: unchecked