Should psutil be a runtime dependency?

The production server module imports psutil at module load ([src/petals/server/server.py:9-15](https://github.com/bigscience-workshop/petals/blob/22afba627a7eb4fcfe9418c49472c6a51334b8ac/src/petals/server/server.py#L9-L15)) and uses it to determine host memory ([src/petals/server/server.py:294-299](https://github.com/bigscience-workshop/petals/blob/22afba627a7eb4fcfe9418c49472c6a51334b8ac/src/petals/server/server.py#L294-L299)).

However, psutil is now only listed under [options.extras_require]. This means a clean production installation (e.g., pip install petals) will not include it, causing the server to crash immediately on startup with an ImportError.

**Proposed Fix**
Move psutil from the extra to the core _install_requires_ list in setup.cfg.

Read setup.cfg and compare psutil's current extras_require entry with the core install_requires list. Check the imports and host-memory usage in src/petals/server/server.py:9-15 and 294-299, then verify that a clean pip install petals includes psutil and the production server no longer fails with ImportError.

Written by the indexing model from the issue text.