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>

.gitignore

@@ -31,3 +31,10 @@ node_modules/
 .idea/
 .vscode/
 .DS_Store
+
+# Python / Django backend
+.venv/
+__pycache__/
+*.pyc
+db.sqlite3
+/media/