REDHAT-BUG-2516143: Medium severity Red Hat Quay vulnerability

A flaw was found in Red Hat Quay's Stripe billing webhook handler. The /webhooks/stripe endpoint at endpoints/webhooks.py accepts incoming JSON requests without validating the Stripe-Signature header, allowing an unauthenticated attacker to forge billing events. The endpoint is registered unconditionally, even when FEATUREBILLING is disabled. An attacker can forge charge.succeeded events to reset a namespace's build quota to the server-configured maximum and trigger unsolicited billing emails (invoice, payment-failed, subscription-change) to namespace administrators. The checkout.session.completed path calls stripe.SetupIntent.retrieve with attacker-controlled IDs, but subsequent mutations use values from Stripe's response rather than the attacker's payload. Impact is Medium-High for quay.io deployments with real Stripe integration and Low for self-hosted defaults using FakeStripe.