Feature idea: sync models to backends

[johnbrownlow]

When using multiple backends in a Swarm, have the ability to sync selected models to selected backends. This would be particularly useful when running headless or remote backends. For example, if you have models downloaded from (say) Civitai on the main instance and a Linux instance of Swarm running as a backend, it's actually quite hard to download the matching files on the Linux machine (often requires API authentication and the URLs are non-obvious). It would be much simpler to be able to upload the required files for a generation from the local machine. This could be done manually and/or if a request was made to a backend on which the specific models for a generation were not available.