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/services/status_service.py

@@ -10,21 +10,30 @@ The function mutates the technique but does **not** commit.
 The caller is responsible for committing the session.
 """
 
+# Import Session from sqlalchemy.orm
 from sqlalchemy.orm import Session
 
+# Import TechniqueEntity from app.domain.entities.technique
 from app.domain.entities.technique import TechniqueEntity
+
+# Import Technique from app.models.technique
 from app.models.technique import Technique
 
 
+# Define function recalculate_technique_status
 def recalculate_technique_status(db: Session, technique: Technique) -> None:
     """Recompute ``technique.status_global`` from its tests.
 
     ``db`` is accepted for backward compatibility but is not used
     directly — test data comes from the ORM relationship.
     """
+    # Assign entity = TechniqueEntity.from_orm(technique)
     entity = TechniqueEntity.from_orm(technique)
+    # Assign test_snapshots = [
     test_snapshots = [
         (t.state, t.detection_result) for t in technique.tests
     ]
+    # Call entity.recalculate_status()
     entity.recalculate_status(test_snapshots)
+    # Assign technique.status_global = entity.status_global
     technique.status_global = entity.status_global