feat: backup download endpoint + skip-backup sync option

- Add GET /api/backups/download/{project}/{env}/{name} for browser file saves
- Add Save button on local backups in the UI
- Add skip_backup query param to sync endpoint (skips pre-sync snapshot)
- Add Skip safety backup checkbox in sync modal
- Bump app version to v15

	app/routers/backups.py	diff | view | blame | history
	app/routers/sync_data.py	diff | view | blame | history
	static/index.html	diff | view | blame | history
	static/js/app.js	diff | view | blame | history