Batch emit events to runs (legacy)

Emit multiple events in a single request. All events are stored idempotently.

Note: This is the legacy endpoint. For the newer execution-first API with richer metadata, use POST /v1/events instead.

Each event can specify its own runId, customerId, and workflowId. If runId is provided, the event is attached to that run. Otherwise, it's a standalone event.