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.

An optional commitLimit controls how many commits are examined, up to a server maximum of 2000. Omitting it runs at the default 200-commit 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.