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/example_record_events_test.go

@@ -0,0 +1,34 @@
+package cabana_test
+
+import (
+	"context"
+	"fmt"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/cabana"
+	"git.golem15.com/golem15/summercms/modules/pact"
+)
+
+func ExampleEmitRecordSaved() {
+	app := backpack.New(nil) // in a plugin, the application it booted with
+
+	// acme.audit listens from its Boot step; record events arrive as pointers.
+	app.Events.Listen("acme.audit", func(ctx context.Context, ev *pact.RecordSaved) error {
+		for _, change := range ev.Changes {
+			fmt.Printf("%s %s: %s %q -> %q (locale %q)\n", ev.Model, ev.ID, change.Attribute, change.Old, change.New, change.Locale)
+		}
+		return nil
+	})
+
+	// acme.blog's own API handler has saved a post outside the admin forms.
+	// The write has committed, so it passes no transaction and the event
+	// fires at once; inside a lagoon.Transaction it would pass tx and the
+	// event would wait for the commit.
+	cabana.EmitRecordSaved(context.Background(), app, nil, pact.RecordSaved{
+		Model:   "acme_blog_posts",
+		ID:      "42",
+		Changes: []pact.RecordChange{{Attribute: "slug", Old: "hello", New: "hello-world"}},
+	})
+	// Output:
+	// acme_blog_posts 42: slug "hello" -> "hello-world" (locale "")
+}