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

classes/field_types.go

@@ -10,12 +10,14 @@ import (
 	"git.golem15.com/golem15/summercms/modules/pact"
 )
 
-// The elements of the SEO field types (assets/src/elements).
+// The elements of the SEO field types and widgets (assets/src/elements).
 const (
-	ElementText   = "golem15-seo-text"
-	ElementRobots = "golem15-seo-robots"
-	ElementSERP   = "golem15-seo-serp"
-	ElementImage  = "golem15-seo-image"
+	ElementText         = "golem15-seo-text"
+	ElementRobots       = "golem15-seo-robots"
+	ElementSERP         = "golem15-seo-serp"
+	ElementImage        = "golem15-seo-image"
+	ElementRobotsTxt    = "golem15-seo-robotstxt"
+	ElementRobotsTester = "golem15-seo-robotstester"
 )
 
 // The element bundle every SEO field type loads.
@@ -27,13 +29,18 @@ var (
 // errRobotsInvalid is the refusal of a malformed robots value.
 const errRobotsInvalid = "golem15.seo::lang.robots.invalid"
 
+// errRobotsTooLarge refuses a robots.txt over MaxRobotsBytes.
+const errRobotsTooLarge = "golem15.seo::lang.robots.too_large"
+
 // FieldTypes returns the SEO admin field types:
 //   - seotext and mlseotext (E1): text with a live length counter; option
 //     measure: title (input) or description (textarea);
 //   - seorobots and mlseorobots (E2): the robots directives editor; option
 //     scope: record, rule or type; Decode refuses a malformed value;
 //   - seoimage (E7): an uploaded social image stored in the media bucket,
-//     its URL as the value (D-100); uploads need permission.
+//     its URL as the value (D-100); uploads need permission;
+//   - robotstxt (E4): the robots.txt editor with live lint; Decode refuses
+//     a file over 500 KB (D-22).
 //
 // app is asked for the booted application at request time.
 func FieldTypes(app func() *backpack.App, permission string) []pact.FormFieldType {
@@ -61,9 +68,29 @@ func FieldTypes(app func() *backpack.App, permission string) []pact.FormFieldTyp
 			Scripts: elementScripts, Styles: elementStyles,
 			Decode: decodeImage, Upload: UploadImage(app, permission),
 		},
+		{
+			Name: "robotstxt", Element: ElementRobotsTxt,
+			Scripts: elementScripts, Styles: elementStyles,
+			Decode: decodeRobotsTxt,
+		},
 	}
 }
 
+// decodeRobotsTxt stores a robots.txt with Unix line ends and refuses one
+// over MaxRobotsBytes (robots.too_large); lint findings are warnings in
+// the element and never block a save.
+func decodeRobotsTxt(_ context.Context, in pact.FormFieldValue) (pact.FormFieldDecoded, error) {
+	s, ok := stringValue(in.Value)
+	if !ok {
+		return pact.FormFieldDecoded{}, &pact.FormFieldError{Messages: []string{"The " + in.Field + " field must be text."}}
+	}
+	s = strings.ReplaceAll(s, "\r\n", "\n")
+	if len(s) > MaxRobotsBytes {
+		return pact.FormFieldDecoded{}, &pact.FormFieldError{Messages: []string{errRobotsTooLarge}}
+	}
+	return pact.FormFieldDecoded{Value: s}, nil
+}
+
 func checkMeasure(options map[string]any) error {
 	if raw, ok := options["measure"]; ok {
 		if s, _ := raw.(string); s != "title" && s != "description" {