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>

README.md

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+# scene_analysis
+
+- `tl/` — the lumet frontend (ClojureScript / reagent / re-frame). See `tl/`.
+- Django backend (this folder) — users, admin, and persistence of OTIO + the
+  timeline scene.
+
+## Backend
+
+The backend stores one `Project` per editing session: the uploaded **OTIO**
+source file, the playback **fps**, and the live **scene** (the `{:tracks
+:groups}` mark-group map the frontend keeps in app-db) as a JSON dump.
+
+### Run
+
+```bash
+python -m venv .venv && .venv/bin/pip install -r requirements.txt
+.venv/bin/python manage.py migrate
+DJANGO_SUPERUSER_PASSWORD=admin .venv/bin/python manage.py createsuperuser --noinput --username admin --email admin@example.com
+.venv/bin/python manage.py seed_demo          # demo project from tl/.../one_two_three.otio
+.venv/bin/python manage.py runserver
+```
+
+Admin: <http://127.0.0.1:8000/admin/> (admin / admin). Upload OTIO and inspect
+the scene JSON there.
+
+### API (session auth; 401 if not logged in)
+
+| Method | Path | Purpose |
+| --- | --- | --- |
+| GET | `/api/projects/` | the current user's projects |
+| GET | `/api/projects/<id>/scene/` | `{fps, scene}` |
+| PUT | `/api/projects/<id>/scene/` | replace `scene` and/or `fps` (JSON body) |
+| GET | `/api/projects/<id>/otio/` | serve the uploaded OTIO file |
+
+The frontend currently loads `/one_two_three.otio` and persists annotations to
+localStorage; pointing it at `/api/projects/<id>/otio/` and the scene endpoints
+is the next wiring step (CORS/credentials needed across the dev ports).