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/admin.py

@@ -0,0 +1,11 @@
+from django.contrib import admin
+
+from .models import Project
+
+
+@admin.register(Project)
+class ProjectAdmin(admin.ModelAdmin):
+    list_display = ("name", "owner", "fps", "updated")
+    list_filter = ("owner",)
+    search_fields = ("name",)
+    readonly_fields = ("created", "updated")