Replace the stored floor arrangement

Replaces the whole document, creating the row on the company's first edit (version 1). Every id must belong to the caller's company and two occupants of the SAME team cannot share a slot — either breach is a 400 with an explicit message, never a silent drop. version is an optional optimistic lock: when given and no longer current, 409 Version conflict rather than overwriting a colleague's whole rearrangement.