recompute_ready only scanned 'todo' tasks for promotion, ignoring
'blocked' tasks entirely. When a task was blocked (e.g. by the circuit
breaker) and its parent dependencies later completed, the task stayed
stuck in 'blocked' forever unless manually unblocked.
Now recompute_ready also scans 'blocked' tasks. When all parents are
done/archived, the blocked task is promoted to 'ready' with failure
counters reset — equivalent to an automatic unblock.
Includes a regression test for the blocked-parent-done promotion path.
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