Fix chat/room ordering that could differ between devices (#45)

Two independent gaps, both fixed since the report was ambiguous about
which "chats" meant:

- ChatPane.tsx concatenated history (REST-fetched) and live (WS-pushed)
  without sorting, so anything that could desync receipt order from
  send order -- a rejoin/resync racing a still-in-flight WS message,
  which opening the same room on another device triggers directly via
  a fresh socket connection -- could render messages out of
  chronological order. Now sorted by created_at (stable sort, so
  same-timestamp messages keep their relative order).
- list_member_rooms/list_open_rooms/list_recent_messages ordered by
  created_at alone, with no secondary tiebreaker. Postgres doesn't
  guarantee a stable order for tied rows across separate query
  executions, so two rooms/messages sharing an identical timestamp
  (a real possibility -- rapid sends, bulk-created rooms) could come
  back in a different order on two separate fetches, i.e. two devices.
  Added id as a secondary sort key everywhere this showed up.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/test_ws_chat.py

@@ -64,6 +64,51 @@ def test_ws_join_and_message_roundtrip(ws_client):
     assert any(m["content"] == "hello" and m["username"] == username for m in history)
 
 
+def test_history_tiebreaks_identical_timestamps_by_id(ws_client):
+    # Two messages sharing the exact same created_at (a real possibility --
+    # rapid sends, a webhook, microsecond-precision collisions under load)
+    # must still come back in a single, stable order every call, not
+    # whatever order Postgres feels like giving ties with no secondary sort
+    # key -- see list_recent_messages's order_by comment.
+    username = _unique("alice")
+    _register(ws_client, username=username)
+    room = ws_client.post("/api/rooms", json={"name": _unique("general")}).json()
+
+    with ws_client.websocket_connect("/ws/chat") as ws:
+        ws.send_json({"type": "join", "room_id": room["id"]})
+        assert ws.receive_json()["type"] == "joined"
+        ws.send_json({"type": "message", "room_id": room["id"], "content": "first"})
+        first = ws.receive_json()
+        ws.send_json({"type": "message", "room_id": room["id"], "content": "second"})
+        second = ws.receive_json()
+
+    async def _force_same_timestamp():
+        from sqlalchemy import select
+
+        from app.models import Message
+
+        async with ws_client.session_factory() as session:
+            result = await session.execute(
+                select(Message).where(Message.id.in_([uuid.UUID(first["id"]), uuid.UUID(second["id"])]))
+            )
+            rows = {str(m.id): m for m in result.scalars().all()}
+            rows[first["id"]].created_at = rows[second["id"]].created_at
+            await session.commit()
+
+    ws_client.portal.call(_force_same_timestamp)
+
+    resp = ws_client.get(f"/api/rooms/{room['id']}/messages")
+    assert resp.status_code == 200
+    history = [m for m in resp.json() if m["content"] in ("first", "second")]
+    assert len(history) == 2
+
+    # order_by(created_at.desc(), id.desc()) then reversed for display --
+    # with a tied created_at, the higher id (whichever message that is)
+    # must consistently render second.
+    expected_order = sorted([first, second], key=lambda m: m["id"])
+    assert [m["content"] for m in history] == [m["content"] for m in expected_order]
+
+
 def test_ws_message_without_join_errors(ws_client):
     _register(ws_client, username=_unique("alice"))
     room = ws_client.post("/api/rooms", json={"name": _unique("general")}).json()