feat(32-02): record events from every admin write path

- Delete and each bulk-delete row fire RecordDeleted after commit
- tree reorder fires RecordSaved per moved record (parent/sort in model types)
- relation-manager child create/update/delete fire events naming the
  controller that edits the child's model
- TestRecordEvents covers create, update, translated slug, no-op, rollback
  (validation, FormBeforeUpdate, extension Save), failing listener, reorder,
  child writes, delete and bulk delete; extension Save sees the changes
- docs: Record events in extending.md, admin-controllers notes, cabana README

modules/cabana/relation.go

@@ -10,6 +10,7 @@ import (
 	"slices"
 	"strings"
 
+	"git.golem15.com/golem15/summercms/modules/backpack"
 	"git.golem15.com/golem15/summercms/modules/lagoon"
 	"git.golem15.com/golem15/summercms/modules/pact"
 	"git.golem15.com/golem15/summercms/modules/phrasebook"
@@ -200,6 +201,10 @@ type RelationService struct {
 	bucket *blob.Bucket
 	tr     *phrasebook.Translator
 	writer TranslationWriter
+	// app receives the record events of child writes; reg names the
+	// controller that edits a child's model. Both may be nil.
+	app *backpack.App
+	reg *Registry
 }
 
 func (s RelationSchema) MarshalJSON() ([]byte, error) {