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/migrations/0001_initial.py

@@ -0,0 +1,33 @@
+# Generated by Django 6.0.6 on 2026-06-29 03:38
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Project',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=200)),
+                ('otio', models.FileField(blank=True, null=True, upload_to='otio/')),
+                ('fps', models.FloatField(default=23.976023976023978)),
+                ('scene', models.JSONField(blank=True, default=dict)),
+                ('created', models.DateTimeField(auto_now_add=True)),
+                ('updated', models.DateTimeField(auto_now=True)),
+                ('owner', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='projects', to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'ordering': ['-updated'],
+            },
+        ),
+    ]