Attribute annotations and scene saves to users

Server-authoritative attribution: on each scene PUT the server diffs incoming
annotation groups and stamps createdBy/editedBy (+ timestamps) from request.user,
ignoring client-sent stamps so authorship can't be forged. Each save also writes
a Revision (user, time, +N ~N −N summary, snapshot of the annotation layer),
visible in the admin and via /revisions/. Projects gain collaborators so several
users can edit one project and get distinct attribution.

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

scenes/migrations/0002_revision.py

@@ -0,0 +1,30 @@
+# Generated by Django 6.0.6 on 2026-06-29 03:47
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('scenes', '0001_initial'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Revision',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('created', models.DateTimeField(auto_now_add=True)),
+                ('summary', models.CharField(blank=True, max_length=200)),
+                ('annotations', models.JSONField(blank=True, default=dict)),
+                ('project', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='revisions', to='scenes.project')),
+                ('user', models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='+', to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'ordering': ['-created'],
+            },
+        ),
+    ]