✨(backend) add connection-test API

Currently users have no way to reliably test their connection before
joining a room. To address this, we plan to build a connection-test
page.

The testing requires a dedicated LiveKit token, issued without going
through the room API, which is tied to registered meetings, lobby
rules, and longer-lived access tokens.

Introduce a new viewset for all diagnostics-related features. The
first route issues a token for diagnostics, even for anonymous
users. Each request creates a new dedicated room so users never
share the same LiveKit room during tests. Tokens are short-lived
(default 10 minutes) to limit reuse, and the endpoint is throttled
to prevent abuse.

A Celery worker also schedules a callback that deletes the room
after a certain delay, in every case.

src/backend/core/api/feature_flag.py

@@ -17,6 +17,7 @@ class FeatureFlag:
         "addons": "ADDONS_ENABLED",
         "application": "APPLICATION_ENABLED",
         "roomkit": "ROOMKIT_ENABLED",
+        "connection_test": "CONNECTION_TEST_ENABLED",
     }
 
     @classmethod