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/infrastructure/persistence/repositories/__init__.py

@@ -1,8 +1,13 @@
+"""Concrete SQLAlchemy repository implementations."""
+# Import  from app.infrastructure.persistence.repositories.sa_technique_repository
 from app.infrastructure.persistence.repositories.sa_technique_repository import (
     SATechniqueRepository,
 )
+
+# Import  from app.infrastructure.persistence.repositories.sa_test_repository
 from app.infrastructure.persistence.repositories.sa_test_repository import (
     SATestRepository,
 )
 
+# Assign __all__ = ["SATechniqueRepository", "SATestRepository"]
 __all__ = ["SATechniqueRepository", "SATestRepository"]