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.