I built my own UptimeRobot in a weekend with Next.js 16 + Vercel Cron

I've been paying UptimeRobot for years. It works. The free tier is generous. I have no real beef with them. But every time I added a 6th monitor, the upgrade modal appeared. Every time I logged in to check a site, the dashboard nudged me toward Pro. Every time I wanted a public status page on my own domain, that was a paid feature too. Eventually I asked the question every indie dev asks at some point: how hard could this actually be? It turned out: a weekend to MVP, two weeks to ship to paying