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/tasks.py

@@ -1,6 +1,6 @@
 """Task and cache management endpoints."""
 
-from datetime import datetime
+from datetime import UTC, datetime
 
 from fastapi import APIRouter
 
@@ -91,9 +91,9 @@ async def get_active_tasks():
                 try:
                     started_at = datetime.fromisoformat(timestamp_str.replace("Z", "+00:00"))
                 except (ValueError, AttributeError):
-                    started_at = datetime.utcnow()
+                    started_at = datetime.now(UTC)
             else:
-                started_at = datetime.utcnow()
+                started_at = datetime.now(UTC)
 
             active_downloads.append(
                 models.ActiveDownloadTask(