✨(backend) add endpoint to update a participant role during a meeting

Add an endpoint that allows updating a user's role while in a
meeting. The goal is to let users promote other connected
participants to admin or moderator, so the burden of administrating
a meeting can be shared.

src/backend/core/services/room_roles.py

@@ -0,0 +1,178 @@
+"""Room role management service.
+
+Single entry point for changing a user's role on a room, used by:
+- the in-meeting endpoint (promote/demote a connected participant)
+- (more to come soon)
+
+`ResourceAccess` is the source of truth. The LiveKit `room_admin`
+participant attribute is only a projection of it, synced best-effort.
+"""
+
+from logging import getLogger
+from uuid import UUID
+
+from core import models
+from core.services.participants_management import (
+    ParticipantNotFoundException,
+    ParticipantsManagement,
+    ParticipantsManagementException,
+)
+
+logger = getLogger(__name__)
+
+
+class RoomRoleError(Exception):
+    """Base exception for room role management errors."""
+
+    status_code = 400
+
+
+class SelfActionError(RoomRoleError):
+    """Raised when a user tries to change their own role."""
+
+    status_code = 403
+
+
+class OwnerRoleError(RoomRoleError):
+    """Raised when trying to demote an owner or grant ownership."""
+
+    status_code = 403
+
+
+class ParticipantNotInMeetingError(RoomRoleError):
+    """Raised when the target participant is not connected to the meeting."""
+
+    status_code = 404
+
+
+class UserNotFoundError(RoomRoleError):
+    """Raised when the target participant has no user account in database."""
+
+    status_code = 404
+
+
+ASSIGNABLE_ROLES = (models.RoleChoices.MEMBER, models.RoleChoices.ADMIN)
+
+
+class RoomRoleService:
+    """Manage promotion and demotion of room co-hosts."""
+
+    def set_role(
+        self, room: models.Room, user: models.User, role: str, actor: models.User
+    ):
+        """Persist `role` for `user` on `room`, idempotently and atomically.
+
+        Returns the up-to-date `ResourceAccess`. Never grants or removes
+        ownership: granting OWNER is refused, and an existing OWNER access
+        is never modified.
+        """
+
+        if role not in ASSIGNABLE_ROLES:
+            raise OwnerRoleError("Ownership cannot be granted through this action.")
+
+        if actor is not None and user == actor:
+            raise SelfActionError("You cannot change your own role.")
+
+        access, created = models.ResourceAccess.objects.get_or_create(
+            resource=room,
+            user=user,
+            defaults={"role": role},
+        )
+
+        if created:
+            return access
+
+        if access.role == models.RoleChoices.OWNER:
+            raise OwnerRoleError("Room owners cannot be demoted.")
+
+        if access.role != role:
+            access.role = role
+            access.save(update_fields=["role", "updated_at"])
+
+        return access
+
+    def set_participant_role(
+        self,
+        room: models.Room,
+        participant_identity: UUID,
+        role: str,
+        actor: models.User,
+    ):
+        """Change the role of a participant currently connected to the meeting.
+
+        - The participant must be connected (checked against LiveKit).
+        - The participant must map to a user account.
+        - The role is persisted in DB then mirrored to LiveKit.
+
+        Returns a dict: {"role", "livekit_synced"}.
+        """
+
+        room_name = str(room.pk)
+        participants_management = ParticipantsManagement()
+
+        try:
+            is_in_meeting = participants_management.check_if_in_meeting(
+                room_name=room_name, identity=str(participant_identity)
+            )
+        except ParticipantNotFoundException as e:
+            raise ParticipantNotInMeetingError(
+                "Participant is not connected to this meeting."
+            ) from e
+
+        if not is_in_meeting:
+            raise ParticipantNotInMeetingError(
+                "Participant is not connected to this meeting."
+            )
+
+        user = models.User.objects.filter(sub=participant_identity).first()
+
+        if user is None:
+            raise UserNotFoundError(
+                "This participant has no user account and cannot be assigned a role."
+            )
+
+        # Source of truth first: even if the LiveKit sync below fails,
+        # the role is real and any fresh token will carry it.
+        self.set_role(room=room, user=user, role=role, actor=actor)
+
+        livekit_synced = self._sync_livekit_role(
+            room_name=room_name,
+            participant_identity=str(participant_identity),
+            is_admin=role == models.RoleChoices.ADMIN,
+        )
+
+        return {
+            "role": role,
+            "livekit_synced": livekit_synced,
+        }
+
+    @staticmethod
+    def _sync_livekit_role(room_name: str, participant_identity: str, is_admin: bool):
+        """Mirror the role to the participant's LiveKit attributes.
+
+        Best-effort: returns False on failure instead of raising, so callers
+        can report a partial success. Re-running the action re-syncs.
+        """
+        try:
+            ParticipantsManagement().update(
+                room_name=room_name,
+                identity=participant_identity,
+                attributes={"room_admin": "true" if is_admin else "false"},
+            )
+        except ParticipantNotFoundException:
+            # The participant left between the presence check and the update:
+            # harmless, the DB state (if any) remains authoritative.
+            logger.info(
+                "Participant %s left room %s before role sync",
+                participant_identity,
+                room_name,
+            )
+            return False
+        except ParticipantsManagementException:
+            logger.exception(
+                "Could not sync role to LiveKit for participant %s in room %s",
+                participant_identity,
+                room_name,
+            )
+            return False
+        return True