Webhook timeout from slow endpoint processing

Stripe webhooks fail or timeout when endpoint takes longer than ~500ms to respond with 200 OK. Stripe retries failed webhooks for up to three days in live mode, potentially clogging the system with repeated deliveries.

Decouple webhook processing: (1) Return 200 OK to Stripe immediately within ~500ms, (2) Enqueue event payload to background task runner (Redis, SQS, RabbitMQ), (3) Process webhook asynchronously. Verify webhook signature using endpoint secret. Avoid heavy database writes or email sending before acknowledging receipt.