✨(backend) introduce a token exchange endpoint for iframe embeds

Some integrators render our videoconference inside an iframe, where
our cookie-based authentication does not work: our cookies are
SameSite=Lax/Strict, so the iframe drops them.

We looked at what Jitsi offers: a shared secret used to sign JWTs
that authenticate users coming from external services. Since we
already expose an external API where third parties authenticate as
a given user, it was simpler for us to add an exchange mechanism on
top of that.

Flow:

* Through the external API, mint a short-lived, single-use exchange
  code for a user.
* The third party hands that code to the frontend as a URL fragment.
* The frontend exchanges the code for a longer-lived JWT that can be
  used to query the regular API viewsets.

Known limitations and follow-ups:

* At some point it would be nice to shorten the JWT lifetime and
  add a refresh mechanism. This will be handled in a follow-up PR
  when actually needed.
* CSP rules to control which origins are allowed to embed the app
  in an iframe still need to be added.
* This alternative authentication cannot easily be scoped to a
  subset of endpoints without adding a lot of complexity, so it is
  accepted globally on the API for now.

src/backend/core/api/viewsets.py

@@ -75,6 +75,7 @@ from core.recording.worker.mediator import (
     WorkerServiceMediator,
 )
 from core.services.invitation import InvitationService
+from core.services.jwt_token import JwtTokenService
 from core.services.livekit_events import (
     LiveKitEventsService,
     LiveKitWebhookError,
@@ -99,6 +100,7 @@ from core.services.room_roles import (
     RoomRoleService,
 )
 from core.services.subtitle import SubtitleException, SubtitleService
+from core.services.transit_code import TransitCodeService
 from core.tasks.connection_test import delete_connection_test_room
 from core.tasks.file import process_file_deletion
 from core.utils import generate_token
@@ -237,6 +239,74 @@ class UserViewSet(
             self.serializer_class(request.user, context=context).data
         )
 
+    @decorators.action(
+        detail=False,
+        methods=["post"],
+        url_path="exchange-access-token",
+        permission_classes=[],
+        throttle_classes=[throttling.ExchangeAccessTokenAnonRateThrottle],
+    )
+    @FeatureFlag.require("user_access_token")
+    def exchange_access_token(self, request):
+        """Exchange a single-use transit code for a user access token.
+
+        The endpoint is unauthenticated: the transit code itself, an opaque
+        random string obtained through the external API and delivered to
+        the embedded frontend via a URL fragment, is the credential. Each
+        code can be exchanged exactly once (consuming it deletes it from
+        the cache); replaying a consumed code is denied and logged.
+
+        The issued JWT authenticates the user the code was minted for on
+        the whole core API, exactly like a session cookie would (similar
+        to lib-jitsi-meet's token authentication), and never appears in
+        any URL. Role-based permissions apply unchanged.
+        """
+        serializer = serializers.TransitCodeSerializer(data=request.data)
+        serializer.is_valid(raise_exception=True)
+
+        code_data = TransitCodeService().consume_code(serializer.validated_data["code"])
+
+        if code_data is None:
+            logger.warning("Invalid, expired or already used transit code")
+            raise drf_exceptions.PermissionDenied(
+                "Invalid, expired or already used transit code."
+            )
+
+        # Re-check the user at exchange time so that a deactivation after
+        # the transit code was minted is taken into account.
+        try:
+            user = models.User.objects.get(id=code_data["user_id"], is_active=True)
+        except models.User.DoesNotExist as excpt:
+            raise drf_exceptions.PermissionDenied(
+                "This account can no longer access the application."
+            ) from excpt
+
+        token_service = JwtTokenService(
+            secret_key=settings.USER_ACCESS_TOKEN_SECRET_KEY,
+            algorithm=settings.USER_ACCESS_TOKEN_ALG,
+            issuer=settings.USER_ACCESS_TOKEN_ISSUER,
+            audience=settings.USER_ACCESS_TOKEN_AUDIENCE,
+            expiration_seconds=settings.USER_ACCESS_TOKEN_TTL,
+            token_type=settings.USER_ACCESS_TOKEN_TYPE,
+        )
+
+        data = token_service.generate_jwt(
+            user,
+            "user:access",
+            {
+                "client_id": code_data.get("client_id", "unknown"),
+            },
+        )
+
+        # Log for auditing
+        logger.info(
+            "User access token issued from transit code: user_id=%s, client_id=%s",
+            user.id,
+            code_data.get("client_id", "unknown"),
+        )
+
+        return drf_response.Response(data)
+
 
 class RoomViewSet(
     mixins.CreateModelMixin,