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

plugin.go

@@ -4,6 +4,7 @@ import (
 	"embed"
 	"fmt"
 	"io/fs"
+	"log/slog"
 	"net/http"
 	"strconv"
 	"time"
@@ -74,7 +75,11 @@ func (p *Plugin) wireSiteCache(app *backpack.App) error {
 	if purger == nil {
 		return nil
 	}
-	return app.Publish(purger)
+	if err := app.Publish(purger); err != nil {
+		return err
+	}
+	slog.Default().Info("journal: site cache purge enabled", "endpoint", purger.Endpoint())
+	return nil
 }
 
 func (p *Plugin) LangFS() fs.FS { return langFS }