Replace read-check-write in task_queue.assign_task with UPDATE...WHERE
status='pending' and decide on rowcount so two concurrent workers
cannot both claim the same task. Also harden complete_task with the
same CAS pattern (status='assigned') so a worker + stuck-handler
double-complete leaves the DB in one state.
The xfail guard in test_worker_task_resilience now passes (4/4).
P1 deepening (issue #7):
S1: is the single source of truth for time-slot parsing,
slot-due checks, session deficit, priority, attention reason, and
high-severity detection. and now delegate
their internal helpers to while keeping the same signatures
(tests continue to pass via the thin wrappers).
S4: encapsulates the stuck-cron
task settlement (fail current task + enqueue retry).
calls it instead of the previous runtime import of .
No observable behaviour change — 873 passed + 5 xfailed unchanged.
