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/site_cache.go

@@ -6,27 +6,28 @@ import (
 	"git.golem15.com/golem15/sm-journal-plugin/classes"
 	"git.golem15.com/golem15/summercms/modules/backpack"
 	"git.golem15.com/golem15/summercms/modules/cabana"
+	"gorm.io/gorm"
 )
 
-// purgeSiteCache registers a site cache purge for one saved entity to run
-// after the save transaction commits. Without a published
-// *classes.SiteCachePurger (no purge configured) or outside a save
-// transaction it does nothing.
-func purgeSiteCache(ctx context.Context, app func() *backpack.App, kind, slug string) {
+// siteCachePurger returns the published *classes.SiteCachePurger and the
+// save transaction of ctx, for classes.PurgeSiteCacheAfterCommit. Without a
+// purger (no purge configured) or outside a save transaction it returns a
+// nil purger, which PurgeSiteCacheAfterCommit ignores.
+func siteCachePurger(ctx context.Context, app func() *backpack.App) (*classes.SiteCachePurger, *gorm.DB) {
 	if app == nil {
-		return
+		return nil, nil
 	}
 	a := app()
 	if a == nil {
-		return
+		return nil, nil
 	}
 	purger, ok := a.Lookup[*classes.SiteCachePurger]()
 	if !ok || purger == nil {
-		return
+		return nil, nil
 	}
 	tx, ok := cabana.TxFromContext(ctx)
 	if !ok {
-		return
+		return nil, nil
 	}
-	classes.PurgeSiteCacheAfterCommit(ctx, tx, purger, kind, slug)
+	return purger, tx
 }