Start evaluation run

Request body for POST /api/v1/evaluations.

Response body for POST /api/v1/evaluations.

The POST endpoint returns immediately with status="pending"; the actual evaluation runs as a background asyncio.create_task so the event loop stays free to serve other requests (VAL-SEC-009, VAL-SEC-011). Clients poll GET /api/v1/evaluations/{run_id} to observe the pending -> running -> completed transition (VAL-SEC-010).