Attribute annotations and scene saves to users

Server-authoritative attribution: on each scene PUT the server diffs incoming
annotation groups and stamps createdBy/editedBy (+ timestamps) from request.user,
ignoring client-sent stamps so authorship can't be forged. Each save also writes
a Revision (user, time, +N ~N −N summary, snapshot of the annotation layer),
visible in the admin and via /revisions/. Projects gain collaborators so several
users can edit one project and get distinct attribution.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scenes/urls.py

@@ -6,4 +6,5 @@ urlpatterns = [
     path("projects/", views.projects),
     path("projects/<int:pk>/scene/", views.scene),
     path("projects/<int:pk>/otio/", views.otio),
+    path("projects/<int:pk>/revisions/", views.revisions),
 ]