Trigger configured webhook

Public webhook receiver. Looks up the configured webhook by its trigger UUID, applies the configured field mappings to the incoming payload, builds a marker, and publishes it through the ingestion pipeline. If the webhook is disabled or has no field mappings yet, the raw payload is stored as the lastReceivedPayload (so dashboard users can configure mappings against a real sample) and the response is { status: 'captured' | 'disabled' }. Successful publishes respond { status: 'ok' }; publish failures return 502.