TaskScheduler.start() aborts stale TaskRun rows but never advanced
ScheduledTask.next_run. Across a restart the in-process _executing set
is empty, so the first post-restart _check_due_tasks() call dispatches
every task whose next_run is still in the past — and so does every
subsequent poll, until the task's regular _execute_task path finally
runs compute_next_run and pushes it forward.
start() now queries active tasks with next_run < now and pushes each
one to now + 60s. The first poll after restart sees them as not-yet-due,
the task runs once normally, and compute_next_run puts the schedule
back on its real cadence. Paused and not-yet-due tasks are left alone.
The validator test was rewritten as a regression test asserting the
opposite of the bug it originally demonstrated, plus two narrower cases
to lock down the filter (only active+overdue is touched).
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