✨(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/authentication.py

@@ -0,0 +1,65 @@
+"""Authentication for the roomkit API of the Meet core app."""
+
+import logging
+import secrets
+
+from django.conf import settings
+
+from rest_framework.authentication import BaseAuthentication
+from rest_framework.exceptions import AuthenticationFailed
+
+from core.recording.event.authentication import MachineUser
+
+logger = logging.getLogger(__name__)
+
+
+class ServerToServerAuthentication(BaseAuthentication):
+    """Custom authentication class for roomkit server-to-server requests.
+
+    Validates the Authorization header against the roomkit server-to-server
+    token. A valid PIN code is intentionally not enough to authenticate: the
+    endpoints are restricted to the LiveKit SIP module's credentials.
+    """
+
+    AUTH_HEADER = "Authorization"
+    TOKEN_TYPE = "Bearer"  # noqa S105
+
+    def authenticate(self, request):
+        """Validate the Bearer token from the Authorization header.
+
+        Returns a (MachineUser, token) pair on success, and raises
+        AuthenticationFailed if the header is missing, malformed, or contains
+        an invalid token.
+        """
+        required_token = settings.ROOMKIT_SERVER_TO_SERVER_API_TOKEN
+        if not required_token:
+            raise AuthenticationFailed("Server-to-server token is not configured.")
+
+        auth_header = request.headers.get(self.AUTH_HEADER)
+        if not auth_header:
+            logger.warning(
+                "Roomkit authentication failed: missing Authorization header (ip: %s)",
+                request.META.get("REMOTE_ADDR"),
+            )
+            raise AuthenticationFailed("Authorization header is missing.")
+
+        # Validate token format and existence
+        auth_parts = auth_header.split(" ")
+        if len(auth_parts) != 2 or auth_parts[0] != self.TOKEN_TYPE:
+            raise AuthenticationFailed("Invalid authorization header.")
+
+        token = auth_parts[1]
+
+        # Use constant-time comparison to prevent timing attacks
+        if not secrets.compare_digest(token.encode(), required_token.encode()):
+            logger.warning(
+                "Roomkit authentication failed: invalid token (ip: %s)",
+                request.META.get("REMOTE_ADDR"),
+            )
+            raise AuthenticationFailed("Invalid server-to-server token.")
+
+        return MachineUser(username="roomkit"), token
+
+    def authenticate_header(self, request):
+        """Return the WWW-Authenticate header value."""
+        return f"{self.TOKEN_TYPE} realm='Roomkit server to server'"