✨(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/roomkit/serializers.py

@@ -0,0 +1,21 @@
+"""Serializers for the roomkit API of the Meet core app."""
+
+# pylint: disable=abstract-method
+
+from django.conf import settings
+
+from rest_framework import serializers
+
+from core.api.serializers import BaseValidationOnlySerializer
+
+
+class RoomKitJoinSerializer(BaseValidationOnlySerializer):
+    """Validate roomkit join requests from the LiveKit SIP module."""
+
+    pin_code = serializers.CharField(required=True)
+
+    def validate_pin_code(self, value):
+        """Ensure the PIN code matches the configured length."""
+        if len(value) != settings.ROOM_TELEPHONY_PIN_LENGTH:
+            raise serializers.ValidationError("PIN code length is invalid.")
+        return value