feat(admin): edit category content in the admin form behind the HTML gate

- add a content textarea (size huge) after description, with en and pl phrases
- add content to Category.Fillable
- export classes.CheckHTML and run it from FormBeforeCreate and FormBeforeUpdate:
  unsafe category content answers 422 on content, blank content is stored as NULL

controllers/categories.go

@@ -1,6 +1,11 @@
 package controllers
 
 import (
+	"context"
+	"errors"
+	"strings"
+
+	"git.golem15.com/golem15/sm-journal-plugin/classes"
 	"git.golem15.com/golem15/sm-journal-plugin/models"
 	"git.golem15.com/golem15/summercms/modules/backpack"
 	"git.golem15.com/golem15/summercms/modules/cabana"
@@ -40,9 +45,46 @@ func (categoriesAdminController) AdminFieldRelations() []cabana.FieldRelationCon
 	}}
 }
 
+func (categoriesAdminController) FormBeforeCreate(ctx context.Context, model any) error {
+	cat, ok := model.(*models.Category)
+	if !ok || cat == nil {
+		return errors.New("categories admin: form model is not a category")
+	}
+	return prepareCategoryWrite(cat)
+}
+
+func (categoriesAdminController) FormBeforeUpdate(ctx context.Context, model any) error {
+	cat, ok := model.(*models.Category)
+	if !ok || cat == nil {
+		return errors.New("categories admin: form model is not a category")
+	}
+	return prepareCategoryWrite(cat)
+}
+
+// prepareCategoryWrite gates the category HTML body (D-21): blank content is
+// stored as NULL, and content that fails classes.CheckHTML is a 422 on
+// content. Any other content is stored exactly as submitted.
+func prepareCategoryWrite(cat *models.Category) error {
+	if cat.Content == nil {
+		return nil
+	}
+	if strings.TrimSpace(*cat.Content) == "" {
+		cat.Content = nil
+		return nil
+	}
+	if err := classes.CheckHTML(*cat.Content); err != nil {
+		return &cabana.ValidationError{Details: map[string]any{
+			"content": []string{err.Error()},
+		}}
+	}
+	return nil
+}
+
 var (
 	_ pact.AdminController         = categoriesAdminController{}
 	_ pact.AdminPermissioned       = categoriesAdminController{}
 	_ pact.AdminRecordSource       = categoriesAdminController{}
+	_ pact.FormBeforeCreate        = categoriesAdminController{}
+	_ pact.FormBeforeUpdate        = categoriesAdminController{}
 	_ cabana.FieldRelationProvider = categoriesAdminController{}
 )