Migrate the backend from pip to uv

Replace requirements.txt with a uv workspace and lockfile, install the five tavolo packages from packages/* in development and non-editable in the image, and build the Docker image with uv. Move dev tools to a PEP 735 dependency group and refresh the locked dependency versions.

server/packages/tavolo-briscola/README.md

@@ -92,8 +92,8 @@ the match), so the hand-ack timeout is accepted but unused.
 ## Development (from `server/`)
 
 ```sh
-.venv/bin/python -m unittest discover -s packages/tavolo-briscola/tests
-.venv/bin/python -m mypy -p tavolo.briscola
+uv run python -m unittest discover -s packages/tavolo-briscola/tests
+uv run mypy -p tavolo.briscola
 ```
 
 `test_engine.py`/`test_plugin.py` cover the two classic variants;