Flaky-alert validation now reads from a marker table instead of scanning raw runs

The flaky-alert validation gate now uses a dedicated marker table to check whether a test case has passed on the default branch. The marker table is fed by a new materialized view that records one row per validated test case, so the validation query becomes a fast primary-key lookup instead of scanning billions of raw test-case runs. This release also fixes the rollout so the migration completes quickly and no longer blocks the production deploy.