Start re-deriving the project's Priority Orbit from its open tickets

Starts one AI pass over the project's OPEN tickets to REPLACE the stored derivation and answers 202 immediately with the new run's id — the model pass itself runs detached from this request and can take minutes on a large project. Poll GET /prioritization/progress for the real phase; GET /prioritization for the finished derivation. Manual only — nothing re-derives on a schedule or on read. Error codes: PRIORITIZATION_NOT_CONFIGURED (503, no Anthropic API key available), NO_OPEN_TICKETS (409, nothing to prioritize), PRIORITIZATION_RUNNING (409, a run is already in flight for this project). A background model failure (PRIORITIZATION_FAILED) surfaces as a failed row on GET /prioritization/progress, not as an HTTP response from this endpoint.