I shipped a paid SaaS in 3 days with no user accounts: the billing setup and 3 bugs that bit me

I write a small blog on the side. Every time I picked a topic I hit the same wall: Ahrefs and Semrush are $130+ a month, and even when I had the numbers, "volume 590, difficulty 17" didn't tell me whether to write the post or skip it. So I built Keyword Brief. You type a keyword and get a plain-language GO / MAYBE / SKIP, real Google volume and...