🚧(backend) introduce a lobby system

Implement lobby service using cache as LiveKit doesn't natively support
secure lobby functionality. Their teams recommended to create our own
system in our app's backend.

The lobby system is totally independant of the DRF session IDs,
making the request_entry endpoint authentication agnostic.

This decoupling prevents future DRF changes from breaking lobby functionality
and makes participant tracking more explicit.

Security audit is needed as current LiveKit tokens have excessive privileges
for unprivileged users. I'll offer more option ASAP for the admin to control
participant privileges.

Race condition handling also requires improvements, but should not be critical
at this point.

A great enhancement, would be to add a webhook, notifying the backend when the
room is closed, to reset cache.

This commit makes redis a prerequesite to run the suite of tests. The readme
and CI will be updated in dedicated commits.

src/backend/core/models.py

@@ -404,6 +404,11 @@ class Room(Resource):
             raise ValidationError({"name": f'Room name "{self.name:s}" is reserved.'})
         super().clean_fields(exclude=exclude)
 
+    @property
+    def is_public(self):
+        """Check if a room is public"""
+        return self.access_level == RoomAccessLevel.PUBLIC
+
 
 class BaseAccessManager(models.Manager):
     """Base manager for handling resource access control."""