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/routes/profiles.py

@@ -4,7 +4,7 @@ import io
 import json as _json
 import logging
 import tempfile
-from datetime import datetime
+from datetime import UTC, datetime
 from pathlib import Path
 
 from fastapi import APIRouter, Depends, File, Form, HTTPException, UploadFile
@@ -356,7 +356,7 @@ async def update_profile_effects(
     else:
         profile.effects_chain = None
 
-    profile.updated_at = datetime.utcnow()
+    profile.updated_at = datetime.now(UTC)
     db.commit()
     db.refresh(profile)