I15-1: Do not allow reording of backgrounds

For i15-1, reordering the backgrounds is probably not required and will produce unusual behaviour on the queue as it may try and re-queue backgrounds. We should probably lock backgrounds so they are not reorderable and show this to a user in a nice way. This is probably best implemented by the queue tagging it as unorderable and the UI reacting

Start by tracing how the queue represents and reorders backgrounds, then find the UI code that renders reorder controls. Confirm how I15-1 backgrounds are identified and how the queue exposes ordering capability. Done means those backgrounds cannot be reordered and the UI communicates that restriction clearly without re-queueing them.

Written by the indexing model from the issue text.