feat: add AirShelf core implementation

core/backend/apps/common/api.py

@@ -0,0 +1,23 @@
+from rest_framework.exceptions import PermissionDenied
+
+
+def get_current_team(user):
+    membership = user.team_memberships.filter(status="active").select_related("team").first()
+    if not membership:
+        raise PermissionDenied("current user has no active team")
+    return membership.team
+
+
+class TeamScopedViewSetMixin:
+    team_field = "team"
+
+    def get_team(self):
+        return get_current_team(self.request.user)
+
+    def get_queryset(self):
+        queryset = super().get_queryset()
+        return queryset.filter(**{self.team_field: self.get_team()})
+
+    def perform_create(self, serializer):
+        serializer.save(team=self.get_team(), created_by=self.request.user)
+