Spec: Stripe billing

Add Stripe to this kernel. Do not copy a frozen SDK tutorial from 2025.

Do this

npx skills add stripe/ai@stripe-best-practices

Then follow .agents/skills/add-stripe/SKILL.md.

Already in this repo

• subscriptions in db/schema.ts (tier, status, period columns)
• Account page explains billing is an add-on
• No stripe package until you install it

Map onto the kernel

• Add stripeCustomerId and stripeSubscriptionId on subscriptions
• npm run db:push
• Checkout / portal from app/account/actions.ts
• Webhook app/api/webhooks/stripe/route.ts with signature verification
• Keep a no-keys local path