Poll a backtest job

Min tier: free. Returns the job's current status (queued → running → done | error), the validated params it was enqueued with, and — once done — the full result (summary metrics, equity curve, per-trade rows, documented assumptions).

Ownership is enforced against the Clerk user that owns the presenting key. A job id belonging to another account returns 404, not 403, so the endpoint never confirms that an id exists.