✨(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/urls.py

@@ -30,6 +30,11 @@ router.register(
     addons_viewsets.SessionViewSet,
     basename="addons_sessions",
 )
+router.register(
+    "diagnostics",
+    viewsets.DiagnosticsViewSet,
+    basename="diagnostics",
+)
 
 # - External API
 external_router = SimpleRouter()