Closes issue #135.
Once a runner claimed a queued job, the API still showed its old waiting reason (seen on doomsdayone/metalgear run issue #164: 'no runner can take it: earth-1 does not hold image…' on a running job). The claim's UPDATE now clears waiting.
Test: images_are_sent_by_chunk_and_offered_to_runners_that_fetch now checks a claimed job has no waiting reason. Hub gate green (170 tests, clippy, fmt). Bookmark is main's heads plus this change.