How I Built 19 Per-Topic OG Images with Japanese Fonts at Build Time (Next.js + Satori)

The Problem Our team runs bitcoin.ne.jp — a free, bilingual Bitcoin education library with 19 deep-dive topics. Every learn topic was sharing the same generic Open Graph image. Click-through-rate on social shares was flat, and the SERP previews looked interchangeable. I wanted one OG image per top