refactor: update module docstrings for clarity and consistency

backend/app/models/base.py

@@ -1,3 +1,5 @@
+"""Base model mixins and shared SQLModel abstractions."""
+
 from __future__ import annotations
 
 from typing import ClassVar, Self
@@ -8,4 +10,6 @@ from app.db.query_manager import ManagerDescriptor
 
 
 class QueryModel(SQLModel, table=False):
+    """Base SQLModel with a shared query manager descriptor."""
+
     objects: ClassVar[ManagerDescriptor[Self]] = ManagerDescriptor()