How to build a credit system for a Next.js AI app (Stripe + Supabase)

If you're building an AI app (image generation, transcription, an agent, anything that calls a model) you've probably realized a flat "$10/month" doesn't work. Every action costs you real money in GPU/API spend, so a single power user can torch your margins. The answer is usage credits: users buy a balance, each action spends some. Credits sound trivial. They are not. I've shipped about 10 small AI/SaaS apps, and the credit layer is where I got burned every single time. It took three patterns t...