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/models.py

@@ -0,0 +1,25 @@
+from django.conf import settings
+from django.db import models
+
+NTSC_24 = 24000 / 1001
+
+
+class Project(models.Model):
+    """One editing project: an uploaded OTIO source + the live mark-group scene
+    (tl.scene) the frontend reads and writes. The scene is just a JSON dump for
+    now — the whole {:tracks :groups} map the client keeps in app-db."""
+
+    owner = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE,
+                              related_name="projects")
+    name = models.CharField(max_length=200)
+    otio = models.FileField(upload_to="otio/", blank=True, null=True)
+    fps = models.FloatField(default=NTSC_24)
+    scene = models.JSONField(default=dict, blank=True)
+    created = models.DateTimeField(auto_now_add=True)
+    updated = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        ordering = ["-updated"]
+
+    def __str__(self):
+        return f"{self.name} (#{self.pk})"