fix: sqlalchemy sessions per request

Best practice for web-apps is to have a session per web request.

Instead of having a per worker-thread, we add a middleware that
generates a unique ID per request, utilizing scoped_sessions
scopefunc (custom function for defining a session scope)
in combination with a ContextVar.
With this we create a new session per request.

Signed-off-by: moson <moson@archlinux.org>

test/test_routes.py

@@ -102,6 +102,7 @@ def test_user_language(client: TestClient, user: User):
         req.cookies = {"AURSID": sid}
         response = req.post("/language", data=post_data)
     assert response.status_code == int(HTTPStatus.SEE_OTHER)
+    db.refresh(user)
     assert user.LangPreference == "de"