Function mark_done_batch 

pub fn mark_done_batch(
    backend: &mut Backend,
    reports: &[TaskReport],
) -> Result<(), String>

Persists a batch of finished reports to the Task store, with bounded retry on a transient DB failure. The finalize thread drains the batch off the bounded done channel and hands it here in one mark_done call (amortizing the round-trip). On exhausted retries it returns Err, which the finalize thread propagates into a panic → the manager aborts the whole dispatcher — the intended fail-fast on a DB runaway (the channel design replaces the old mutex-poisoning that achieved the same propagation). A crash here loses nothing: the tasks remain Queued and recover on restart.