TL;DR I built Arac Saglik Karnesi (Vehicle Health Card), a free digital service-history tool for auto repair shops in Turkey. Every vehicle gets an 8-character code. Customers type that code into a page and see the full maintenance history: no signup, no app, no password. The interesting engineering problem wasn't the lookup. It was making one shared Next.js app safely serve dozens of independent auto shops without any of them ever seeing another shop's data. Two audiences, two a...