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.

backend/app/domain/value_objects/__init__.py

@@ -1,4 +1,9 @@
+"""Immutable domain value objects."""
+# Import MitreId from app.domain.value_objects.mitre_id
 from app.domain.value_objects.mitre_id import MitreId
+
+# Import ScoringWeights from app.domain.value_objects.scoring_weights
 from app.domain.value_objects.scoring_weights import ScoringWeights
 
+# Assign __all__ = ["MitreId", "ScoringWeights"]
 __all__ = ["MitreId", "ScoringWeights"]