Streaming events

SSE event types, auth, and stream.end semantics.

curl -N -H "X-SWARM-API-KEY: $SWARM_API_KEY" \
  http://localhost:8000/api/swarm/runs/<run_id>/events

Event types: run.started, step.completed, approval.required, cost.update, stream.end.

Auth required when SWARM_API_KEY is set.