ci(ebook-search): run tests via uv on the runner instead of docker

The gitea runner containers have no docker access, so build the test
env with uv from the existing lockfile and run pytest directly:

- test_ebook_search workflow: uv sync --locked + uv run pytest, with
  UV_PYTHON_DOWNLOADS=never so uv uses the nix-provided python 3.14
- add uv to the runner hostPackages (needs a jeeves rebuild to apply)
- ignore nested **/.venv in .dockerignore (uv sync creates one in
  python/ebook_search/docker)
- document the uv test commands in the docker README; the docker test
  image remains for validating the image itself

systems/jeeves/runners/nix_builder.nix

@@ -138,6 +138,7 @@ in
                 nixos-rebuild
                 nodejs
                 treefmt
+                uv
                 wget
               ];
             };