Rework Stripe billing from flat plans to usage-based pricing

Flat-rate pricing is capping our revenue. Our API monitoring product charges $49/99/249 a month regardless of volume, and our biggest customers send 50x the traffic of our smallest. We are moving to a base-plus-usage model: a platform fee plus metered billing on API checks, with volume tiers. Concretely: model the new pricing in Stripe (Products, Prices, metered usage records), build the migration path for ~800 existing subscriptions including grandfathering rules we will define together, handle proration edge cases, and rebuild our in-app billing page so customers can see usage against their tier in near real time. Backend is Node.js with TypeScript; the usage events already flow through our queue, so metering data is available. You should have shipped metered billing on Stripe before and have scars from it. We want to hear what went wrong last time and how you would avoid it here. Weekly milestone check-ins, code review by our CTO.