Align plot_numeric_distributions module path with tests

## Summary

The new `plot_numeric_distributions` helper added in PR #145 lives under `apps/ml/visualizations/data_distribution.py`, but the associated tests in `tests/test_distribution.py` currently import from a backend utilities path. This causes `pytest` to fail with `ModuleNotFoundError` once tests are run against `main`.

## Tasks

- Decide the canonical location for the data distribution utility:
- Either keep it under `apps/ml/visualizations/data_distribution.py`, **or**
- Move it under a backend utilities namespace (e.g. `apps/backend/utils/visualization/data_distribution.py`).
- Update `tests/test_distribution.py` to import `plot_numeric_distributions` from the chosen canonical module path.
- Run `pytest tests/test_distribution.py` and ensure the tests pass.

## Acceptance Criteria

- `pytest tests/test_distribution.py` runs successfully without import errors.
- The visualization module and tests both reference the same, well-defined import path.
- No breaking changes to the public API of `plot_numeric_distributions`.