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/admin_registry.go

@@ -32,6 +32,7 @@ func AdminControllers(app func() *backpack.App) []pact.AdminController {
 		redirectsAdminController{app: app},
 		notFoundAdminController{app: app},
 		rulesAdminController{app: app},
+		robotsAdminController{app: app},
 		historyAdminController{app: app},
 	}
 }