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},
 	}
 }

controllers/robots.go

@@ -0,0 +1,101 @@
+package controllers
+
+import (
+	"context"
+	"errors"
+
+	"git.golem15.com/golem15/sm-seo-plugin/classes"
+	"git.golem15.com/golem15/sm-seo-plugin/models"
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/pact"
+	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
+)
+
+// RobotsControllerID is the robots.txt screen (UI-SPEC C10).
+const RobotsControllerID = "golem15.seo.robots"
+
+// robotsAdminController edits the robots.txt singleton (D-22, D-24,
+// D-25). Its form holds the staging callout and the served file (P4); the
+// editor (E4), the tester (E5) and the AI bot switches come from the
+// plugin's own form extension on this controller, so the tester can read
+// the unsaved editor text and switches.
+type robotsAdminController struct {
+	app func() *backpack.App
+}
+
+func (robotsAdminController) ID() string        { return RobotsControllerID }
+func (robotsAdminController) ModelName() string { return `Golem15\SEO\Models\Robots` }
+func (robotsAdminController) ConfigDir() string { return "controllers/robots" }
+func (robotsAdminController) NewRecord() any    { return &models.Robots{} }
+func (robotsAdminController) RequiredPermissions() []string {
+	return []string{PermissionManage}
+}
+
+// SingletonRecord returns robots row 1, creating it when the migration's
+// row is gone (first or create, safe against a concurrent first request).
+func (robotsAdminController) SingletonRecord(_ context.Context, db *gorm.DB) (any, error) {
+	if db == nil {
+		return nil, errors.New("golem15.seo: database is not configured")
+	}
+	var rows []models.Robots
+	if err := db.Where("id = ?", 1).Limit(1).Find(&rows).Error; err != nil {
+		return nil, err
+	}
+	if len(rows) > 0 {
+		return &rows[0], nil
+	}
+	row := models.Robots{ID: 1, Bots: models.BotSwitches{}}
+	if err := db.Clauses(clause.OnConflict{DoNothing: true}).Create(&row).Error; err != nil {
+		return nil, err
+	}
+	if err := db.Where("id = ?", 1).Limit(1).Find(&rows).Error; err != nil || len(rows) == 0 {
+		return nil, errors.New("golem15.seo: robots row is missing")
+	}
+	return &rows[0], nil
+}
+
+// StagingView is the staging callout partial's view model.
+type StagingView struct {
+	Staging bool
+}
+
+// ServedView is P4: the file crawlers get and, outside production, the
+// production version under it.
+type ServedView struct {
+	Served     string
+	Production string
+	Staging    bool
+}
+
+// PartialData serves the staging callout and P4.
+func (c robotsAdminController) PartialData(ctx context.Context, name string, _ any) (any, error) {
+	svc, err := classes.NewService(appOf(c.app))
+	if err != nil {
+		return nil, err
+	}
+	staging := svc.Staging()
+	switch name {
+	case "staging":
+		return StagingView{Staging: staging}, nil
+	case "robots_served":
+		production, err := svc.ServedFile(ctx, true)
+		if err != nil {
+			return nil, err
+		}
+		view := ServedView{Served: production, Staging: staging}
+		if staging {
+			view.Served, view.Production = classes.NonProductionRobots, production
+		}
+		return view, nil
+	}
+	return nil, nil
+}
+
+var (
+	_ pact.AdminController   = robotsAdminController{}
+	_ pact.AdminPermissioned = robotsAdminController{}
+	_ pact.AdminRecordSource = robotsAdminController{}
+	_ pact.AdminSingleton    = robotsAdminController{}
+	_ pact.AdminPartialData  = robotsAdminController{}
+)

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