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・in