What I never skip, even in the smallest build:
✅ A clean database schema. It's the hardest thing to change once real data is in it.
✅ Background jobs. Emails, API calls and reports run outside the request, so a slow service never freezes your app.
✅ Error logging and alerts, so you hear about a bug before your users do.
✅ Login and payments done properly. Users trust you with these before anything else.