✨(backend) add connection-test API

Currently users have no way to reliably test their connection before
joining a room. We then want to create a connection test page
to adress this issue.

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

We introduce a new API endpoint GET /api/v1.0/connection-test/
to issue a dedicated token for diagnostics, even for anonymous users.
Each request creates a new 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.

src/backend/core/api/throttling.py

@@ -73,3 +73,15 @@ class CreationCallbackAnonRateThrottle(MonitoredAnonRateThrottle):
     """Throttle Anonymous user requesting room generation callback"""
 
     scope = "creation_callback"
+
+
+class ConnectionTestUserRateThrottle(MonitoredUserRateThrottle):
+    """Throttle authenticated users requesting connection test tokens."""
+
+    scope = "connection_test"
+
+
+class ConnectionTestAnonRateThrottle(MonitoredAnonRateThrottle):
+    """Throttle anonymous users requesting connection test tokens."""
+
+    scope = "connection_test"