Chore: Refactor device assignment to use single controller method

[Steve-Mcl]

Once #4336 is merged we should look to replace the old code for assigning and unassigning devices originally embedded within in the API route to use the new bulk method in the devices controller.

See comments and details in the original thread

Originally posted by @Steve-Mcl in https://github.com/FlowFuse/flowfuse/issues/4336#issuecomment-2273430520