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/models.py

@@ -15,6 +15,7 @@ class Project(models.Model):
                                            related_name="shared_projects")
     name = models.CharField(max_length=200)
     otio = models.FileField(upload_to="otio/", blank=True, null=True)
+    clip = models.FileField(upload_to="clips/", blank=True, null=True)
     fps = models.FloatField(default=NTSC_24)
     scene = models.JSONField(default=dict, blank=True)
     created = models.DateTimeField(auto_now_add=True)