Fix deploy blocked by validated-on-branch backfill

Server deploys no longer fail because of a long-running migration backfill. The test_case_runs_validated_on_branch migration now creates only the marker table and materialized view in a sub-second step, with historical rows backfilled out-of-band after deploy instead of blocking the rollout. Users benefit from faster flaky-test detection (the validation query reads the marker table) and from server upgrades completing on schedule.