fix(packaging): name a build backend that exists

[OfficialAbhinavSingh]

The problem

build-backend = "setuptools.backends.legacy:build"

There is no such module. Checked against setuptools 84.0.0:

setuptools.build_meta: EXISTS
setuptools.backends:   DOES NOT EXIST

The legacy shim was never called setuptools.backends at any version, so this has never worked. The package could not be built or installed at all:

ModuleNotFoundError: No module named 'setuptools.backends'

uv build and pip install . both fail on it. That is also how it surfaced — uv run inside the project directory installs the project first, and fell over before running anything.

After

$ uv build
Successfully built dist/sql_optim_env-2.0.0.tar.gz
Successfully built dist/sql_optim_env-2.0.0-py3-none-any.whl

$ uv run python -c "print('ok')"
ok

Scope

One line. Nothing else depended on it working: the Space image installs requirements.txt and then COPYs the tree, so it never pip-installed the package and never hit this. Kept separate from #1 for that reason — that PR is about what the image installs and which port it declares, this is about whether the package can be built at all.

[OfficialAbhinavSingh]

Landed on main as f0c3572 (rebased onto bb76742 from #1, signature preserved).

GitHub shows this as Closed rather than Merged because the commit reached main via a local fast-forward before the branch ref was updated — the change itself is in:

$ git show origin/main:pyproject.toml | grep build-backend
build-backend = "setuptools.build_meta"