Send a synthesized test event through a customer-owned webhook

Enqueues a fixture-shaped sample payload for the given channel (or the webhook's first channel when omitted). Uses the same delivery worker as real events — same HMAC signing, same retry schedule, same `webhook_deliveries` history. The synthesized payload always carries `test: true` and a unique `eventId: test_<uuid>` so receivers can distinguish it from real events.