feat(spare-parts): stamp assignedTo on the register work order server side

[rob-aslanian]

Summary

• registerSparePart/registerSparePartsBatch already received assignedTo in the request body (the FE was sending it), but only used it for the spare-part record itself — the auto-created register work order never got it, so the FE had to make a separate assignWorkOrder call per created WO afterward.
• Both handlers now also copy assignedTo onto the register WO's info at creation time, when provided. For the batch handler, the shared WO takes it from the first part (all parts in one submission carry the same assignee).
• Removes the need for a follow-up assign call entirely — one atomic write instead of register-then-assign.