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/apps.py

@@ -0,0 +1,5 @@
+from django.apps import AppConfig
+
+
+class ScenesConfig(AppConfig):
+    name = 'scenes'