Setting `rate_limit=0` for AI generations should disable the feature for all

From #2588, we got a rate limit setting in the CMS panel. Setting it to `0` currently raises an error. We should change this to make it disable the feature for all users. This may be handy in cases where there's a connection instability with the AI providers and we need to apply some contingence.

Start in the CMS panel implementation and trace the rate_limit setting introduced by issue #2588 through its validation and AI-generation gate. Confirm that setting it to 0 no longer raises an error and that AI generations are disabled for all users; preserve the existing behavior for positive limits.

Written by the indexing model from the issue text.