feat(06-02): create Go→TypeScript event constant codegen tool

- Add backend/events/cmd/genevents/main.go using go/ast for deterministic output
- Add //go:generate directive to backend/events/events.go
- Generate frontend/src/events.ts with all 21 constants including LibraryConfigChanged
- Atomic file writes via temp file + rename
- Comment groups preserved with trailing period stripping

backend/events/events.go

@@ -3,6 +3,8 @@
 // the corresponding event names in the TypeScript frontend.
 package events
 
+//go:generate go run ./cmd/genevents -source events.go -output ../../frontend/src/events.ts
+
 // Playback events (backend → frontend push).
 const (
 	PlaybackStateChanged = "PlaybackStateChanged"