Trigger a verifier backtest for a context lineage

Starts a backtest for the lineage’s current version: every verifier on that version is evaluated against the source repo’s recent commit history, producing a pass/fail/na result per commit per verifier. One backtest covers all of the version’s verifiers.

The number of commits examined is fixed by the server, so the request takes no size or limit. Responds 202 with the backtest’s ids when started. Responds 409 with the in-progress backtest when one is already running for the same version.