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-mo <mo-son@mailbox.org>

test/test_trusted_user_routes.py

@@ -764,6 +764,7 @@ def test_tu_proposal_vote(client, proposal):
     assert response.status_code == int(HTTPStatus.OK)
 
     # Check that the proposal record got updated.
+    db.refresh(voteinfo)
     assert voteinfo.Yes == yes + 1
 
     # Check that the new TUVote exists.