Add update (CRUD) for events and people; record the full-CRUD invariant

Events and people are now editable, not write-once: PATCH /events/{id} (type, structured date, place, notes) and PATCH /persons/{id} (vitals, privacy, and the primary name's given/surname). CLAUDE.md gains rule #8: every stored object must support full CRUD in API and UI — historical research is constant correction. Tests cover both updates.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/tests/test_core_api.py

@@ -68,6 +68,25 @@ async def test_public_tree_viewable_but_not_editable_by_non_member(client):
     assert resp.status_code == 403
 
 
+async def test_person_update(client):
+    token = await register(client, "edit@example.com")
+    h = auth(token)
+    tid = (await client.post("/api/v1/trees", json={"name": "T"}, headers=h)).json()["id"]
+    pid = (
+        await client.post(
+            f"/api/v1/trees/{tid}/persons", json={"given": "Jon", "surname": "Smith"}, headers=h
+        )
+    ).json()["id"]
+    resp = await client.patch(
+        f"/api/v1/trees/{tid}/persons/{pid}",
+        json={"given": "John", "gender": "male"},
+        headers=auth(token),
+    )
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["primary_name"] == "John Smith"
+    assert resp.json()["gender"] == "male"
+
+
 async def test_auth_required_without_token(client):
     resp = await client.get("/api/v1/trees")
     assert resp.status_code == 401