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/export_import.py

@@ -432,7 +432,7 @@ async def import_generation_from_zip(file_bytes: bytes, db: Session) -> dict:
                     duration=generation_data["duration"],
                     seed=generation_data.get("seed"),
                     instruct=generation_data.get("instruct"),
-                    created_at=datetime.utcnow(),
+                    created_at=datetime.now(UTC),
                 )
                 
                 db.add(db_generation)