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>

server/urls.py

@@ -1,4 +1,6 @@
 """URL configuration for the scene_analysis backend."""
+from django.conf import settings
+from django.conf.urls.static import static
 from django.contrib import admin
 from django.urls import include, path
 
@@ -6,3 +8,7 @@ urlpatterns = [
     path("admin/", admin.site.urls),
     path("api/", include("scenes.urls")),
 ]
+
+# Serve uploaded OTIO/clip files in development (supports HTTP range → video seek).
+if settings.DEBUG:
+    urlpatterns += static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)