✨(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/viewsets.py

@@ -0,0 +1,89 @@
+"""Roomkit API endpoints for meeting-room (SIP) device integration."""
+
+from logging import getLogger
+
+from rest_framework import decorators, viewsets
+from rest_framework import (
+    exceptions as drf_exceptions,
+)
+from rest_framework import (
+    response as drf_response,
+)
+from rest_framework import (
+    status as drf_status,
+)
+
+from core import analytics, models
+from core.api import permissions, throttling
+from core.api.feature_flag import FeatureFlag
+from core.services.telephony import TelephonyException, TelephonyService
+
+from . import authentication, serializers
+
+logger = getLogger(__name__)
+
+
+class RoomKitViewSet(viewsets.ViewSet):
+    """Server-to-server API endpoints for the roomkit integration.
+
+    Groups all interactions between roomkit (SIP) devices and the backend,
+    brokered by the LiveKit SIP module. All endpoints are authenticated
+    with the roomkit server-to-server tokens.
+    """
+
+    authentication_classes = [authentication.ServerToServerAuthentication]
+    permission_classes = [permissions.IsAuthenticated]
+
+    @decorators.action(
+        detail=False,
+        methods=["post"],
+        url_path="join",
+        throttle_classes=[throttling.RoomKitJoinRateThrottle],
+    )
+    @FeatureFlag.require("roomkit")
+    def join(self, request):
+        """Prepare a room for a meeting-room (SIP) device joining by PIN code.
+
+        Called by the LiveKit SIP module when a meeting-room device dials in
+        with a PIN code before any WebRTC participant has joined. Resolves the
+        room by PIN and creates its SIP dispatch rule, so the device can enter
+        without waiting for a WebRTC user.
+
+        The webhook-based creation path is kept: both converge on the same rule
+        through the shared TelephonyService.
+        """
+
+        serializer = serializers.RoomKitJoinSerializer(data=request.data)
+        serializer.is_valid(raise_exception=True)
+
+        try:
+            room = models.Room.objects.get(
+                pin_code=serializer.validated_data["pin_code"]
+            )
+        except models.Room.DoesNotExist as e:
+            raise drf_exceptions.NotFound("No room found for this PIN code.") from e
+
+        try:
+            created = TelephonyService().ensure_dispatch_rule(room)
+        except TelephonyException as e:
+            raise drf_exceptions.APIException("Could not create dispatch rule.") from e
+
+        analytics.capture(
+            request.user,
+            analytics.AnalyticsEvent.ROOMKIT_JOINED,
+            {
+                "room_id": str(room.pk),
+                "dispatch_rule_created": created,
+            },
+        )
+
+        logger.info(
+            "Roomkit join requested: room_id=%s, dispatch_rule_created=%s",
+            room.id,
+            created,
+        )
+
+        return drf_response.Response(
+            {"status": "success"},
+            status=drf_status.HTTP_200_OK,
+        )