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/schemas/notification.py

@@ -1,28 +1,45 @@
 """Pydantic schemas for Notification endpoints."""
 
+# Import uuid
 import uuid
+
+# Import datetime from datetime
 from datetime import datetime
 
+# Import BaseModel, ConfigDict from pydantic
 from pydantic import BaseModel, ConfigDict
 
 
+# Define class NotificationOut
 class NotificationOut(BaseModel):
     """Notification returned by the API."""
 
+    # id: uuid.UUID
     id: uuid.UUID
+    # user_id: uuid.UUID
     user_id: uuid.UUID
+    # type: str
     type: str
+    # title: str
     title: str
+    # Assign message = None
     message: str | None = None
+    # Assign entity_type = None
     entity_type: str | None = None
+    # Assign entity_id = None
     entity_id: uuid.UUID | None = None
+    # Assign read = False
     read: bool = False
+    # Assign created_at = None
     created_at: datetime | None = None
 
+    # Assign model_config = ConfigDict(from_attributes=True)
     model_config = ConfigDict(from_attributes=True)
 
 
+# Define class UnreadCountOut
 class UnreadCountOut(BaseModel):
     """Simple counter response."""
 
+    # unread_count: int
     unread_count: int