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/profiles.py

@@ -4,7 +4,7 @@ import json as _json
 import logging
 import shutil
 import uuid
-from datetime import datetime
+from datetime import UTC, datetime
 from pathlib import Path
 
 from sqlalchemy import func
@@ -183,8 +183,8 @@ async def create_profile(
         design_prompt=data.design_prompt,
         default_engine=default_engine,
         personality=data.personality,
-        created_at=datetime.utcnow(),
-        updated_at=datetime.utcnow(),
+        created_at=datetime.now(UTC),
+        updated_at=datetime.now(UTC),
     )
 
     db.add(db_profile)
@@ -244,7 +244,7 @@ async def add_profile_sample(
 
     db.add(db_sample)
 
-    profile.updated_at = datetime.utcnow()
+    profile.updated_at = datetime.now(UTC)
 
     db.commit()
     db.refresh(db_sample)
@@ -403,7 +403,7 @@ async def update_profile(
     profile.personality = data.personality
     if data.default_engine is not None:
         profile.default_engine = data.default_engine or None  # empty string → NULL
-    profile.updated_at = datetime.utcnow()
+    profile.updated_at = datetime.now(UTC)
 
     db.commit()
     db.refresh(profile)
@@ -672,7 +672,7 @@ async def upload_avatar(
     process_avatar(image_path, str(output_path))
 
     profile.avatar_path = config.to_storage_path(output_path)
-    profile.updated_at = datetime.utcnow()
+    profile.updated_at = datetime.now(UTC)
 
     db.commit()
     db.refresh(profile)
@@ -703,7 +703,7 @@ async def delete_avatar(
         avatar_path.unlink()
 
     profile.avatar_path = None
-    profile.updated_at = datetime.utcnow()
+    profile.updated_at = datetime.now(UTC)
 
     db.commit()