The roster: a PDF goes in, a live schedule comes out
The fortnight's roster is uploaded as it always was. The portal reads it, publishes it as a live schedule, and notifies anyone whose week changed. Staff see their own shifts weeks ahead, put a shift up for swap or fill-in, and the portal refuses a double booking, a second site on the same day, or a shift that lands on a casual who said they were not available. Every version of every week is kept, so there is one answer to which roster is current.