Keeping their cell and spaces they use voluntarily clean: not forced labor, just housekeeping. Keeping common spaces clean: forced labor, the operating of the prison itself is at the expense of the state.
If prisoners are allowed to do any of that work they should be compensated at market rates to make sure there is no unfair competition.
Obviously you couldn't apply a similar definition of "forced labor" to non-prisoners. So you and I are in agreement that you need some special case handling of prisoners in constitutional prohibitions against slavery, as the US and German constitutions recognize. My point simply is that the Constitution is fine as written and these sorts of distinctions should be made in statutory law where stuff like the meaning of "normal housekeeping routine" can be litigated and refined in ordinary rather than constitutional litigation.
Their output should be priced at near or market rates. (There should be no incentive to using prison labor as a way to get cheap labor) The individual's earnings should be reflective of the local market. They're not able to purchase the same goods as the outside world, nor are the prices reflective of the outside world. (I'm not an expert on that but I have a hard time beliving cigs are $10 a pop in prision)
The consequences of prisoners contributing to unfair competition is one of the major plot lines in Shawshank Redemption.