feat(16.2-07): purge the host site cache on every admin post and category write

- posts: FormAfterCreate/Update purge after the content_html translations,
  new FormAfterDelete; categories: FormAfterCreate and FormAfterDelete
- NewSiteCachePurger refuses half configuration, non-http(s) URLs, user
  info, fragments, http to a non-loopback host and tokens under 32 chars;
  Boot logs the endpoint host and path once
- TestPostsAdminPurgesSiteCache smoke test
- README: Configuration section (keys, contract, timing, scope), API rows

controllers/posts.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"strconv"
 
+	"git.golem15.com/golem15/sm-journal-plugin/classes"
 	"git.golem15.com/golem15/sm-journal-plugin/console"
 	"git.golem15.com/golem15/sm-journal-plugin/models"
 	"git.golem15.com/golem15/summercms/modules/backpack"
@@ -226,16 +227,41 @@ func preparePostWrite(ctx context.Context, post *models.Post) error {
 }
 
 // FormAfterCreate writes content_html for every non-default locale whose
-// content the save wrote (D-02).
+// content the save wrote (D-02), then registers the host site cache purge
+// for after the commit.
 func (c postsAdminController) FormAfterCreate(ctx context.Context, model any) error {
-	return c.writeContentHTMLTranslations(ctx, model)
+	return c.afterPostWrite(ctx, model)
 }
 
 // FormAfterUpdate writes content_html for every non-default locale whose
 // content the save wrote (D-02). Untouched locales are not listed and keep
-// their content_html.
+// their content_html. It then registers the host site cache purge for after
+// the commit; an untouched save purges too, which is harmless.
 func (c postsAdminController) FormAfterUpdate(ctx context.Context, model any) error {
-	return c.writeContentHTMLTranslations(ctx, model)
+	return c.afterPostWrite(ctx, model)
+}
+
+// FormAfterDelete registers the host site cache purge for after the delete
+// commits.
+func (c postsAdminController) FormAfterDelete(ctx context.Context, model any) error {
+	post, ok := model.(*models.Post)
+	if !ok || post == nil {
+		return errors.New("posts admin: form model is not a post")
+	}
+	purger, tx := siteCachePurger(ctx, c.app)
+	classes.PurgeSiteCacheAfterCommit(ctx, tx, purger, "post", post.Slug)
+	return nil
+}
+
+func (c postsAdminController) afterPostWrite(ctx context.Context, model any) error {
+	if err := c.writeContentHTMLTranslations(ctx, model); err != nil {
+		return err
+	}
+	if post, ok := model.(*models.Post); ok && post != nil {
+		purger, tx := siteCachePurger(ctx, c.app)
+		classes.PurgeSiteCacheAfterCommit(ctx, tx, purger, "post", post.Slug)
+	}
+	return nil
 }
 
 // writeContentHTMLTranslations stores each written non-default content