Send a synthesized test event through ANY org's webhook (admin)

Cross-tenant operator preview. Synthesizes a fixture-shaped payload for the given channel (or the webhook's first channel) and enqueues it through the standard WebhookDeliveryWorker pipeline — same signing, same retry schedule, same `webhook_deliveries` history. Logged to audit as `admin.webhook.test` (the customer-facing /v1/me variant does NOT audit-log).