Stripe Webhook Was Silently Failing for 5 Days: The 4xx Retry Trap and the Beginning-of-Month Time Bomb

TL;DR 21 invoice.paid webhooks failed for 5 straight days in production. We only noticed because Stripe sent a "we'll auto-disable this endpoint by 5/10" warning email. Root cause: a DB integrity gap caused our handler to throw HttpException(BAD_REQUEST) 竊・Stripe treats 4xx as retry-eligible 竊・infinite retry loop. Lesson : Stripe webhook 4xx is not "client error, give up." It's "please try again." DB lookup misses should be console.warn + 200 OK . Bonus lesson: invoice.paid only fires on subscri