The prune read-lock wrapped all of processItem, including external fetches under
their own timeout. Since a pending RWMutex writer blocks new readers, one prune
arriving behind a slow provider stalled every subsequent item in both drain
pools. Extract persist() so the lock covers only the window it protects: store
placement plus the two row writes.
The in-flight set guarded against a queue row appearing twice in one batch, but
artwork_queue's primary key makes that impossible, drains are serial per pool,
and the pools' kind lists are disjoint. Removing it also retires the cancellation
unwind loop that existed only to release those claims.
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