Reopen Item Reimbursement

Undo a "Mark as Reimbursed" — put a settled payout back in the queue.

Marking a reimbursement paid was a one-way door: returned flipped true, the row dropped out of the Reimbursements queue, and a payout entered against the wrong item (or one whose unit_price turns out to be wrong) had no route back. Both of the guards added alongside this — the 400 in update_material_item and the 400 in void_item_purchase — point here as the way out, so without it those two are dead ends rather than redirections.

Gated to owner/Project Manager via the same role check that guards adding a misc line to an issued PO: handing money back out is a financial decision, and every engineer already holds the generic purchasing:w this endpoint would otherwise be satisfied by.

Body: { reason?: str }