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>

server/urls.py

@@ -0,0 +1,8 @@
+"""URL configuration for the scene_analysis backend."""
+from django.contrib import admin
+from django.urls import include, path
+
+urlpatterns = [
+    path("admin/", admin.site.urls),
+    path("api/", include("scenes.urls")),
+]