refactor(auth): Use user_id as JWT sub and implement get_user_by_id

Switched from username to user_id as the primary identifier in JWT tokens to better support external authentication providers. Added get_user_by_id to HistoryManager and updated API dependencies and tests to reflect these changes.

tests/api/test_persistence.py

@@ -17,7 +17,7 @@ def mock_user():
 @pytest.fixture
 def auth_header(mock_user):
     app.dependency_overrides[get_current_user] = lambda: mock_user
-    token = create_access_token(data={"sub": mock_user.username, "user_id": mock_user.id})
+    token = create_access_token(data={"sub": mock_user.id})
     yield {"Authorization": f"Bearer {token}"}
     app.dependency_overrides.clear()