Replace deprecated datetime.utcnow() with datetime.now(UTC) throughout

Python 3.12 deprecates datetime.utcnow() with a DeprecationWarning and
it will be removed in a future release. Replace all call-site usages in
services, routes, and utils with datetime.now(UTC), and replace the
SQLAlchemy ORM column defaults (which used the bare function reference
datetime.utcnow) with lambda: datetime.now(UTC) so that the returned
objects are timezone-aware and consistent with Python best practice.

Affected: database/models.py, services/{stories,history,profiles,
channels,export_import}.py, routes/{profiles,tasks}.py, utils/tasks.py

backend/services/history.py

@@ -3,7 +3,7 @@ Generation history management module.
 """
 
 from typing import List, Optional, Tuple
-from datetime import datetime
+from datetime import UTC, datetime
 import uuid
 import shutil
 from pathlib import Path
@@ -104,7 +104,7 @@ async def create_generation(
         model_size=model_size,
         status=status,
         source=source,
-        created_at=datetime.utcnow(),
+        created_at=datetime.now(UTC),
     )
 
     db.add(db_generation)