✨(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/analytics/events.py

@@ -8,3 +8,6 @@ class AnalyticsEvent(StrEnum):
 
     # Rooms
     ROOM_CREATED = "room_created"
+
+    # Roomkit (meeting-room SIP devices)
+    ROOMKIT_JOINED = "roomkit_joined"