Fire a Custom Webhook Automation

Fires an automation that has a custom webhook trigger. The token in the URL is the secret: any caller that knows the URL can fire the automation, so this endpoint is unauthenticated by design (no API key). The request body is parsed as JSON and embedded into the automation's prompt under ## Trigger Payload. Non-JSON bodies are wrapped as { "raw": "<body>" }. Bodies larger than 256KB are rejected. Returns 202 Accepted once the execution is queued; disabled automations, unknown tokens, and automations without a custom trigger all return 404 without distinguishing between the three.