Deliver one event using the exact raw UTF-8 JSON bytes covered by the
X-SKUU-Signature header. A 200 response acknowledges the event and ends
retries, including signed permanent validation rejections. Do not retry
401 or 413 unchanged. Retry transport errors and 5xx responses with the
same event_id, exact body bytes and timestamps, for at least three
attempts over at least 15 minutes.

The event discriminator selects exactly one data contract:

• inventory.updated -> InventoryUpdatedEvent
• order.created -> OrderCreatedEvent
• fulfillment.shipped -> FulfillmentShippedEvent
• return.created -> ReturnCreatedEvent