✨(backend) add roomkit viewset to start a room without WebRTC join

Introduce a new viewset that lets the roomkit start a room even when
no WebRTC participant has joined yet.

This is a first entry point that will be extended over time with
more actions a roomkit needs to be able to trigger.

Known limitations:

* The responsibility around SIP rules is currently split between
  the telephony feature and the roomkit one. This may need a
  refactor later on to consolidate ownership in a single place.
* The default throttle might be too low for production usage and
  will likely need to be revisited.

src/backend/core/api/throttling.py

@@ -73,3 +73,15 @@ class CreationCallbackAnonRateThrottle(MonitoredAnonRateThrottle):
     """Throttle Anonymous user requesting room generation callback"""
 
     scope = "creation_callback"
+
+
+class RoomKitJoinRateThrottle(MonitoredUserRateThrottle):
+    """Throttle the LiveKit SIP module requesting roomkit joins.
+
+    The roomkit endpoints are authenticated as a machine user, so all requests
+    share a single throttle bucket. This is not a security measure against
+    brute-force attacks but a guard against accidental hammering from a buggy
+    SIP module.
+    """
+
+    scope = "roomkit_join"