Project create/list API, clip uploads, CORS, public read

- Project gains a clip FileField; POST /api/projects/ creates one from an
  uploaded otio + clip (<=100 MB), owned by the requester
- add /api/me/ and project-detail; scene GET is public (view without login),
  PUT stays owner/collaborator-only
- serve media in DEBUG (range requests for video seek); CORS for the cljs dev
  origin with credentials
- seed_demo now builds the Challengers project from the bundled otio + 480p clip

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scenes/migrations/0004_project_clip.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-06-29 05:39
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('scenes', '0003_project_collaborators'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='project',
+            name='clip',
+            field=models.FileField(blank=True, null=True, upload_to='clips/'),
+        ),
+    ]