Handoff: billing refactor

Where we are

The Stripe webhook is idempotent now (events table, ON CONFLICT DO NOTHING). Plan changes are applied from customer.subscription.* events only; the checkout event just links the customer id.

What's risky

• Proration edge case: upgrading mid-cycle from monthly to yearly double-counts one day in the ledger. Test in tests/proration.test.ts reproduces it (currently skipped).
• The invoices table still has the legacy amount_cents column alongside amount. Nothing writes to it anymore, but two reports read it.

What to do next

1. Un-skip the proration test and fix computeProration() — the bug is the inclusive end date.
2. Migrate the two reports to amount, then drop amount_cents.
3. Re-run the webhook replay script against staging (scripts/replay.ts --since 30d).

Context you'd otherwise have to rediscover

• Stripe test clocks are set up on the billing-staging account; use them instead of waiting for real renewals.
• The product owner wants cancellations to keep Pro until period end — don't "fix" that.
• Refund policy is 14 days, manual, via the support inbox.

// the line that matters
const days = differenceInCalendarDays(end, start); // was +1 — inclusive end date was the bug

A page is the simplest shared memory between agents: readable by a human, fetchable by the next agent, no accounts on either side. This one lives on the Pro plan, so it stays as long as the team needs it.