Two-phase commit of CI job assignment is subject to race condition

Scenario

1. Runner A: requests job, gets job 123
2. Runner A: writes to Redis: job 123 --runner_manager_A
3. Runner B: Requests job, gets job 123 (still in ci_pending_builds!)
4. Runner B: Writes to Redis: job 123 → runner_manager_B (OVERWRITES!)
5. Runner A: DB transaction: remove from ci_pending_builds
6. Runner B: DB transaction: tries to remove (already gone, but no error?)
7. Both runners think they own job 123

Context: !206034 (comment 2828746611)