✨(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/feature_flag.py

@@ -16,6 +16,7 @@ class FeatureFlag:
         "file_upload": "FILE_UPLOAD_ENABLED",
         "addons": "ADDONS_ENABLED",
         "application": "APPLICATION_ENABLED",
+        "roomkit": "ROOMKIT_ENABLED",
     }
 
     @classmethod