Merge scene saves as per-id deltas instead of clobbering

PUT now takes {changed, deleted} and merges per annotation id, so concurrent
edits to different annotations both survive; same-annotation edits are
last-write-wins by arrival. No version/locking. No-op saves don't write a
revision. Tests cover merge, LWW, stale-client safety, attribution
(server-stamped/spoof-resistant, creator preserved), revisions, and access.

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

scenes/tests.py

@@ -1,3 +1,97 @@
-from django.test import TestCase
+import json
 
-# Create your tests here.
+from django.contrib.auth import get_user_model
+from django.test import Client, TestCase
+
+from scenes.models import Project, Revision
+
+User = get_user_model()
+
+
+def ann(name, content="", **extra):
+    return {"type": "annotation", "parent": "root", "name": name,
+            "content": content, "marks": [], **extra}
+
+
+class SceneApiTestCase(TestCase):
+    def setUp(self):
+        self.alice = User.objects.create_user("alice", password="pw")
+        self.bob = User.objects.create_user("bob", password="pw")
+        self.project = Project.objects.create(owner=self.alice, name="p")
+        self.project.collaborators.add(self.bob)
+        self.a = Client(); self.a.login(username="alice", password="pw")
+        self.b = Client(); self.b.login(username="bob", password="pw")
+
+    def put(self, client, **body):
+        return client.put(f"/api/projects/{self.project.pk}/scene/",
+                          json.dumps(body), content_type="application/json")
+
+    def groups(self):
+        self.project.refresh_from_db()
+        return self.project.scene.get("groups", {})
+
+
+class DeltaMergeTests(SceneApiTestCase):
+    def test_different_annotations_merge_without_clobber(self):
+        self.put(self.a, changed={"a1": ann("opening")})
+        self.put(self.b, changed={"a2": ann("beat")})
+        self.assertEqual(set(self.groups()), {"a1", "a2"})
+
+    def test_same_annotation_is_last_write_wins(self):
+        self.put(self.a, changed={"a1": ann("x", "first")})
+        self.put(self.b, changed={"a1": ann("x", "second")})
+        self.assertEqual(self.groups()["a1"]["content"], "second")
+
+    def test_stale_client_neither_resurrects_nor_wipes(self):
+        # bob only ever knew a2; his delta must not erase alice's a1
+        self.put(self.a, changed={"a1": ann("x")})
+        self.put(self.b, changed={"a2": ann("y")})
+        self.assertEqual(set(self.groups()), {"a1", "a2"})
+
+    def test_delete_is_explicit_and_scoped(self):
+        self.put(self.a, changed={"a1": ann("x"), "a2": ann("y")})
+        self.put(self.b, deleted=["a1"])
+        self.assertEqual(set(self.groups()), {"a2"})
+
+
+class AttributionTests(SceneApiTestCase):
+    def test_server_stamps_creator_ignoring_client(self):
+        self.put(self.a, changed={"a1": ann("x", createdBy="hacker", editedBy="hacker")})
+        g = self.groups()["a1"]
+        self.assertEqual((g["createdBy"], g["editedBy"]), ("alice", "alice"))
+
+    def test_edit_preserves_creator_updates_editor(self):
+        self.put(self.a, changed={"a1": ann("x")})
+        self.put(self.b, changed={"a1": ann("x", "edited")})
+        g = self.groups()["a1"]
+        self.assertEqual(g["createdBy"], "alice")
+        self.assertEqual(g["editedBy"], "bob")
+
+    def test_unchanged_annotation_is_not_restamped(self):
+        self.put(self.a, changed={"a1": ann("x")})
+        before = self.groups()["a1"]["editedAt"]
+        self.put(self.b, changed={"a1": ann("x")})           # identical content
+        after = self.groups()["a1"]
+        self.assertEqual(after["editedAt"], before)
+        self.assertEqual(after["editedBy"], "alice")
+
+    def test_revision_per_real_change_only(self):
+        self.put(self.a, changed={"a1": ann("x")})
+        self.put(self.b, changed={"a1": ann("x")})           # no-op → no revision
+        self.put(self.a, deleted=["a1"])
+        revs = Revision.objects.filter(project=self.project).order_by("created")
+        self.assertEqual([r.summary for r in revs],
+                         ["+1 ~0 −0 annotations", "+0 ~0 −1 annotations"])
+        self.assertEqual(revs[0].user, self.alice)
+
+
+class AccessTests(SceneApiTestCase):
+    def test_write_requires_authentication(self):
+        r = Client().put(f"/api/projects/{self.project.pk}/scene/",
+                         json.dumps({"changed": {}}), content_type="application/json")
+        self.assertEqual(r.status_code, 401)
+
+    def test_non_collaborator_cannot_edit(self):
+        User.objects.create_user("carol", password="pw")
+        carol = Client(); carol.login(username="carol", password="pw")
+        self.assertEqual(self.put(carol, changed={"a1": ann("x")}).status_code, 404)