style: use SQLAlchemy 2.0 select() in sync test helpers

Match the select()/scalars() convention already used throughout
app/db/repositories.py instead of the legacy Query API.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/sync/conftest.py

@@ -3,7 +3,7 @@ from typing import Callable
 
 import pytest
 from cryptography.fernet import Fernet
-from sqlalchemy import create_engine
+from sqlalchemy import create_engine, select
 from sqlalchemy.orm import Session, sessionmaker
 from sqlalchemy.pool import StaticPool
 
@@ -68,7 +68,7 @@ def settings(tmp_path: Path) -> StubSettings:
 def load_only_activity(session_factory) -> Callable[[int], Activity]:
     def _load(user_id: int) -> Activity:
         with session_factory() as session:
-            activities = ActivityRepository(session).session.query(Activity).filter_by(user_id=user_id).all()
+            activities = list(session.scalars(select(Activity).where(Activity.user_id == user_id)))
             assert len(activities) == 1, f"expected exactly one activity for user {user_id}, found {len(activities)}"
             return activities[0]