Webhook delivery with backpressure

Design a webhook fan-out system with per-subscriber backpressure. Your platform emits events (e.g. payment.succeeded) and customers register HTTPS endpoints to receive them. Scale: 1M events/sec across 100K subscriber endpoints; some endpoints are fast (10ms), some are slow (5s) or down for hours. One slow/dead subscriber must not delay delivery to healthy ones, and you must retry with backoff, preserve per-subscriber ordering where promised, and guarantee at-least-once delivery.