Last Challenge

Per-run timing (start / finish) for the last-challenge variant.

📄️Record the start of a last-challenge run

Idempotently insert a row into the `last_challenges` table. Re-posting the same (nickname, templateId, startedAt) tuple returns 200 without modifying the existing row.

📄️Record the finish of a last-challenge run

Update the matching `last_challenges` row with `finishedAt` and computed `duration_ms`. Conditional: the row must exist and not already be finished; otherwise responds 404.