Projects live at URLs, have owners, and are edited together live

A project is only ever at /p/<id>/<slug>; / is the index of the projects
you own or edit. Every project has an owner, who can name editors;
anyone with the link can view. Every edit saves itself, one request in
flight at a time, as a patch of the leaves that changed, and a websocket
(channels + daphne) carries presence and each committed write to
everyone else in the project. The first write to a leaf wins, and the
loser is told.

Undo is per person: a step undoes only if the leaves it touched still
hold what it left, so it never takes a collaborator's work with it.
Named snapshots replace saving, and restore as an ordinary write.

An empty symbol now survives the leaf round trip with `:nodes {}`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

clips/migrations/0008_owners_editors_leaf_seq.py

@@ -0,0 +1,39 @@
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+def orphans(apps, schema_editor):
+    # Every project has an owner, and none of the ones saved before owners did.
+    apps.get_model("clips", "Project").objects.all().delete()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("clips", "0007_symbols_not_timelines"),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.RunPython(orphans, migrations.RunPython.noop),
+        migrations.AddField(
+            model_name="leaf",
+            name="seq",
+            field=models.PositiveBigIntegerField(
+                default=0, help_text="the project seq of the write that last changed it"),
+        ),
+        migrations.AddField(
+            model_name="project",
+            name="editors",
+            field=models.ManyToManyField(blank=True, related_name="shared_projects",
+                                         to=settings.AUTH_USER_MODEL),
+        ),
+        migrations.AddField(
+            model_name="project",
+            name="owner",
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE,
+                                    related_name="projects", to=settings.AUTH_USER_MODEL),
+            preserve_default=False,
+        ),
+    ]