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/0003_project_collaborators.py

@@ -0,0 +1,20 @@
+# Generated by Django 6.0.6 on 2026-06-29 04:11
+
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('scenes', '0002_revision'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='project',
+            name='collaborators',
+            field=models.ManyToManyField(blank=True, related_name='shared_projects', to=settings.AUTH_USER_MODEL),
+        ),
+    ]