Task D — Google-style docstrings (Args/Returns) on every public function,
method, and class across all 158 Python files in the backend. Zero ruff D
violations (pydocstyle Google convention).
Task E — Explanatory one-line comment before every code line (~11600 new
comments). ruff check passes clean after isort re-sort.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>