fix(dev): add pytest-env so OPENCV_IO_ENABLE_OPENEXR is set in local pytest runs #242

Problem

pyproject.toml has:

```toml
[tool.pytest.ini_options]
env = ["OPENCV_IO_ENABLE_OPENEXR=1"]
```

…but the `env` key is provided by the `pytest-env` plugin, which is not in `[dependency-groups].dev`. So locally pytest emits:

```
PytestConfigWarning: Unknown config option: env
```

…and any test that hits `cv2.imwrite(*.exr)` before `clip_manager.py` (which sets the env var at import time) fails with:

```
cv2.error: imgcodecs: OpenEXR codec is disabled. You can enable it via 'OPENCV_IO_ENABLE_OPENEXR' option.
```

Currently observable on `tests/test_clip_manager.py::TestVideoMaMa::test_videomama_exr_gamma_handling` when running `pytest` directly (no env var). CI passes because `.github/workflows/ci.yml` sets the env var on the runner.

Fix

One-line change to `pyproject.toml`:

```diff
-dev = ["pytest", "pytest-cov", "ruff", "hypothesis"]
+dev = ["pytest", "pytest-cov", "pytest-env", "ruff", "hypothesis"]
```

After `uv sync --group dev`, pytest picks up the `env` config and OpenEXR works locally without anyone exporting an env var.

Why now

Surfaced while reviewing PR #241 (blue-screen support). Not blocking that PR — happy to send this as a tiny standalone fix.