fix: Simplify query that validates new username already exists

Instead of fetching all users and checking if the new username is present
in the list, we can directly query the database for the username.

backend/endpoints/tests/test_identity.py

@@ -131,6 +131,22 @@ def test_add_user_from_unauthorized_user(
         assert response.status_code == expected_status_code
 
 
+def test_add_user_with_existing_username(client, access_token, admin_user):
+    response = client.post(
+        "/api/users",
+        params={
+            "username": admin_user.username,
+            "password": "new_user_password",
+            "role": Role.VIEWER.value,
+        },
+        headers={"Authorization": f"Bearer {access_token}"},
+    )
+    assert response.status_code == HTTPStatus.BAD_REQUEST
+
+    response = response.json()
+    assert response["detail"] == f"Username {admin_user.username} already exists"
+
+
 def test_update_user(client, access_token, editor_user):
     assert editor_user.role == Role.EDITOR
 

backend/endpoints/user.py

@@ -47,7 +47,8 @@ def add_user(request: Request, username: str, password: str, role: str) -> UserS
             detail="Forbidden",
         )
 
-    if username in [user.username for user in db_user_handler.get_users()]:
+    existing_user = db_user_handler.get_user_by_username(username)
+    if existing_user:
         msg = f"Username {username} already exists"
         log.error(msg)
         raise HTTPException(

backend/handler/database/users_handler.py

@@ -12,11 +12,13 @@ class DBUsersHandler(DBBaseHandler):
         return session.merge(user)
 
     @begin_session
-    def get_user_by_username(self, username: str, session: Session = None):
+    def get_user_by_username(
+        self, username: str, session: Session = None
+    ) -> User | None:
         return session.scalar(select(User).filter_by(username=username).limit(1))
 
     @begin_session
-    def get_user(self, id: int, session: Session = None) -> User:
+    def get_user(self, id: int, session: Session = None) -> User | None:
         return session.get(User, id)
 
     @begin_session