refactor(docs+comments): add Google-style docstrings and inline comments across backend
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>
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