TL;DR — Scale from 1K to 10M Users

You built a SaaS project management tool that just hit product-market fit. Current stack: single DigitalOcean droplet ($80/mo) running a Next.js app, PostgreSQL database on the same server, file uploads stored on the local disk, and sessions in memory. You have 1K monthly active users and the server runs at 30% CPU normally, spiking to 85% during US business hours. Key features: All existing features must continue working at each stage. File uploads must be durable and never lost.