Backfill lab column on historical builds and tests

We should be able to apply backfill to fix previous historical rows to follow the definition #1948.
Populate lab on old historical rows so read paths can switch off JSON extraction without losing data or changing semantics.

## Acceptance criteria
◻ Management command to backfill builds.lab and tests.lab in batches (or a service if we see it more fit).
◻ Options: --batch-size, --since / --until, --dry-run (if necessary we could include more options such as origin).
◻ Idempotent: safe to re-run; only updates rows where column is null or differs from resolved value.