✨(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/api/serializers.py

@@ -312,6 +312,15 @@ class MuteParticipantSerializer(BaseParticipantsManagementSerializer):
     )
 
 
+class ParticipantRoleSerializer(BaseParticipantsManagementSerializer):
+    """Validate an in-meeting role change (promotion/demotion) request."""
+
+    role = serializers.ChoiceField(
+        choices=[models.RoleChoices.MEMBER, models.RoleChoices.ADMIN],
+        help_text="Target role. Ownership cannot be granted this way.",
+    )
+
+
 TrackSource = Literal["camera", "microphone", "screen_share", "screen_share_audio"]