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>

backend/app/domain/enums.py

@@ -5,40 +5,77 @@ truth.  ``models/enums.py`` re-exports them so that existing ORM code
 continues to work without changes.
 """
 
+# Import enum
 import enum
 
 
+# Define class TechniqueStatus
 class TechniqueStatus(str, enum.Enum):
+    """Coverage and evaluation status for a MITRE ATT&CK technique."""
+
+    # Assign not_evaluated = "not_evaluated"
     not_evaluated = "not_evaluated"
+    # Assign in_progress = "in_progress"
     in_progress = "in_progress"
+    # Assign validated = "validated"
     validated = "validated"
+    # Assign partial = "partial"
     partial = "partial"
+    # Assign not_covered = "not_covered"
     not_covered = "not_covered"
+    # Assign review_required = "review_required"
     review_required = "review_required"
 
 
+# Define class TestState
 class TestState(str, enum.Enum):
+    """Lifecycle states in the security test state machine."""
+
+    # Assign draft = "draft"
     draft = "draft"
+    # Assign red_executing = "red_executing"
     red_executing = "red_executing"
+    # Assign blue_evaluating = "blue_evaluating"
     blue_evaluating = "blue_evaluating"
+    # Assign in_review = "in_review"
     in_review = "in_review"
+    # Assign validated = "validated"
     validated = "validated"
+    # Assign rejected = "rejected"
     rejected = "rejected"
 
 
+# Define class TeamSide
 class TeamSide(str, enum.Enum):
+    """Identifies which team (red or blue) an action belongs to."""
+
+    # Assign red = "red"
     red = "red"
+    # Assign blue = "blue"
     blue = "blue"
 
 
+# Define class TestResult
 class TestResult(str, enum.Enum):
+    """Outcome of a red-team test from a detection perspective."""
+
+    # Assign detected = "detected"
     detected = "detected"
+    # Assign not_detected = "not_detected"
     not_detected = "not_detected"
+    # Assign partially_detected = "partially_detected"
     partially_detected = "partially_detected"
 
 
+# Define class DataClassification
 class DataClassification(str, enum.Enum):
+    """Data sensitivity classification levels for compliance and retention policies."""
+
+    # Assign public = "public"
     public = "public"
+    # Assign internal = "internal"
     internal = "internal"
+    # Assign sensitive = "sensitive"
     sensitive = "sensitive"
+    # Assign restricted = "restricted"
     restricted = "restricted"