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-platform/README.md

@@ -58,8 +58,8 @@ examples are [`tavolo-scopone`](../tavolo-scopone/README.md),
 ## Development (from `server/`)
 
 ```sh
-.venv/bin/python -m unittest discover -s packages/tavolo-platform/tests
-.venv/bin/python -m mypy -p tavolo.platform
+uv run python -m unittest discover -s packages/tavolo-platform/tests
+uv run mypy -p tavolo.platform
 ```
 
 Tests run fully in-process against a `DummyEngine` (a two-player toy game