Add Django backend: users, admin, OTIO + scene persistence

A Project (owner, OTIO file, fps, scene JSON) persists the timeline. Session
API: list projects, GET/PUT scene, serve OTIO. Admin manages users/projects;
seed_demo creates a project from the bundled one_two_three.otio.

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

scenes/urls.py

@@ -0,0 +1,9 @@
+from django.urls import path
+
+from . import views
+
+urlpatterns = [
+    path("projects/", views.projects),
+    path("projects/<int:pk>/scene/", views.scene),
+    path("projects/<int:pk>/otio/", views.otio),
+]