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>

requirements.txt

@@ -6,7 +6,9 @@
 # arthur.domain.leaf. No Pillow either; the one thing the backend needs from a PNG
 # is its dimensions, which is a 24-byte header read in clips/blobs.py.
 #
-# Channels arrives with the websocket consumers, which are out of step 9's scope.
+# Channels for the one websocket — presence and broadcast deltas — and daphne to
+# serve it; `daphne` in INSTALLED_APPS makes `runserver` the same server.
 Django>=5.0,<6.0
-gunicorn>=23.0,<24.0
+channels>=4.2,<5.0
+daphne>=4.1,<5.0
 whitenoise>=6.9,<7.0