Re-run all current evaluator bindings against a v2 conversation

v2 alias of POST /v1/sessions/{id}/reevaluate.

The legacy eval endpoint already operates on the conv_* Conversation model and works unchanged — this thin alias keeps the admin UI's path family consistent under /v2/conversations. Wipes existing EvalRuns and re-runs the currently bound evaluators with fresh snapshots; rejects (409) while any run for the conversation is pending / running.