feat: robots.txt editor and tester screen

- golem15.seo.robots singleton (UI-SPEC C10): staging callout and the
  served file (P4); the plugin's own extension adds the editor, the
  tester and the AI bots tab, saved to the robots row with history
- field type robotstxt (E4, live lint, size meter; Decode refuses over
  500 KB) and widget golem15-seo-robotstester (E5) on robots_test, which
  tests the unsaved text and switches through grobotstxt
- saving rules that block a noindex address answers robots.blocks_noindex

controllers/robots/_robots_served.htm

@@ -0,0 +1,10 @@
+{{- with .Data -}}
+<div class="summer-markdown">
+<p>{{ trans "golem15.seo::lang.robots.served_comment" }}</p>
+<pre>{{ .Served }}</pre>
+{{- if .Staging }}
+<p><strong>{{ trans "golem15.seo::lang.robots.served_production" }}</strong></p>
+<pre>{{ .Production }}</pre>
+{{- end }}
+</div>
+{{- end -}}

controllers/robots/_staging.htm

@@ -0,0 +1,6 @@
+{{- if .Data.Staging -}}
+<div class="summer-callout summer-callout--warning" role="status">
+<p class="summer-callout__title">{{ trans "golem15.seo::lang.robots.staging_title" }}</p>
+<p class="summer-callout__text">{{ trans "golem15.seo::lang.robots.staging_text" }}</p>
+</div>
+{{- end -}}

controllers/robots/config_form.yaml

@@ -0,0 +1,13 @@
+# robots.txt singleton (UI-SPEC C10): the singleton footer, no delete.
+name: golem15.seo::lang.robots.title
+form: ~/plugins/golem15/seo/models/robots/fields.yaml
+modelClass: Golem15\SEO\Models\Robots
+defaultRedirect: golem15/seo/robots
+
+update:
+    redirect: golem15/seo/robots
+    redirectClose: golem15/seo/redirects
+
+messages:
+    update: golem15.seo::lang.robots.title
+    saved: backend::lang.messages.form.saved