Stop blocking server deploys on the flaky-alert validation backfill

Fix that unblocks a server deploy by removing the long-running historical backfill from the new ClickHouse marker-table migration. The previous migration for the flaky-alert validation optimization scanned every row of the multi-billion-row test_case_runs table inline, which ran for hours and exceeded the deploy’s migration-hook timeout, failing the production rollout. The new migration creates the marker table and its materialized view only (sub-second), then backfills historical rows out-of-band after deploy. Forward-fill from the materialized view still converges the table as tests run on the default branch, so the underlying CPU win for the flaky-alert validation gate can actually ship to users without blocking the rollout.