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/management/commands/seed_demo.py

@@ -0,0 +1,31 @@
+"""Create a demo Project owned by `admin`, with the bundled OTIO attached."""
+from pathlib import Path
+
+from django.conf import settings
+from django.contrib.auth import get_user_model
+from django.core.files import File
+from django.core.management.base import BaseCommand
+
+from scenes.models import Project
+
+OTIO = settings.BASE_DIR / "tl" / "resources" / "public" / "one_two_three.otio"
+
+
+class Command(BaseCommand):
+    help = "Seed a demo project from the bundled OTIO file."
+
+    def handle(self, *args, **opts):
+        User = get_user_model()
+        owner = User.objects.filter(username="admin").first()
+        if owner is None:
+            self.stderr.write("no 'admin' user — run createsuperuser first")
+            return
+
+        project, created = Project.objects.get_or_create(
+            owner=owner, name="one two three", defaults={"scene": {}})
+        if OTIO.exists() and not project.otio:
+            with OTIO.open("rb") as fh:
+                project.otio.save(Path(OTIO).name, File(fh), save=True)
+
+        verb = "created" if created else "exists"
+        self.stdout.write(f"project #{project.pk} {verb}: {project.name} (otio={bool(project.otio)})")