## Summary
Fixes two related bugs that cause jobs in large workflows (50+ parallel
jobs) to never get a runner assigned even though runners are free.
### Bug 1 — Concurrent runner race
When N runners all poll `FetchTask` with a stale `tasksVersion`
simultaneously, they all query the same waiting job list sorted by
`(updated, id)` and all pick **job #1**. Only one wins the `UPDATE WHERE
task_id=0` optimistic lock; the rest return empty-handed but still
receive `latestVersion` in the response. They then consider themselves
"up to date" and skip `PickTask` on every subsequent poll, leaving jobs
#2–50 permanently unassigned.
**Fix:** `CreateTaskForRunner` now iterates through all matching waiting
jobs. When the optimistic lock fails on job #1, it immediately tries job
#2, then #3, etc., each in its own independent transaction so a failed
attempt rolls back cleanly before the next candidate is tried.
`PickTask` no longer wraps this call in an outer `db.WithTx` (which
caused `halfCommitter` entanglement that prevented per-attempt
rollbacks).
### Bug 2 — Idle runner doesn't re-check after finishing a task
`tasks_version` only bumps when a job transitions **to** waiting (new
workflow triggered, blocked→unblocked). After a runner finishes its
current task it polls `FetchTask` with `tasksVersion == latestVersion`,
so the server skips `PickTask` entirely — the remaining 45 waiting jobs
are invisible to the now-idle runner.
**Fix:** Also call `IncreaseTaskVersion` in `UpdateRunJob` when a
(non-reusable-caller) job transitions to a **done** state. Idle runners
then see a version mismatch on their next poll and attempt `PickTask`,
picking up the remaining jobs.
bircni