refactor: update module docstrings for clarity and consistency

backend/app/core/time.py

@@ -1,3 +1,5 @@
+"""Time-related helpers shared across backend modules."""
+
 from __future__ import annotations
 
 from datetime import UTC, datetime
@@ -5,6 +7,5 @@ from datetime import UTC, datetime
 
 def utcnow() -> datetime:
     """Return a naive UTC datetime without using deprecated datetime.utcnow()."""
-
     # Keep naive UTC values for compatibility with existing DB schema/queries.
     return datetime.now(UTC).replace(tzinfo=None)