feat: add scratchpad field to session model and schemas

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/tests/test_sessions.py

@@ -315,3 +315,51 @@ class TestSessions:
         data = response.json()
         assert len(data) >= 1
         assert all(s["completed_at"] is None for s in data)
+
+    @pytest.mark.asyncio
+    async def test_create_session_has_scratchpad(
+        self, client: AsyncClient, auth_headers: dict, test_tree: dict
+    ):
+        """Test that new sessions include scratchpad field."""
+        session_data = {
+            "tree_id": test_tree["id"],
+        }
+
+        response = await client.post(
+            "/api/v1/sessions",
+            json=session_data,
+            headers=auth_headers
+        )
+
+        assert response.status_code == 201
+        data = response.json()
+        assert "scratchpad" in data
+        assert data["scratchpad"] == ""
+
+    @pytest.mark.asyncio
+    async def test_update_scratchpad_via_put(
+        self, client: AsyncClient, auth_headers: dict, test_tree: dict
+    ):
+        """Test updating scratchpad through the existing PUT endpoint."""
+        # Create session
+        create_response = await client.post(
+            "/api/v1/sessions",
+            json={"tree_id": test_tree["id"]},
+            headers=auth_headers
+        )
+        session_id = create_response.json()["id"]
+
+        # Update scratchpad via PUT
+        update_data = {
+            "scratchpad": "- Server IP: 192.168.1.50\n- Error: 0x80070005"
+        }
+
+        response = await client.put(
+            f"/api/v1/sessions/{session_id}",
+            json=update_data,
+            headers=auth_headers
+        )
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["scratchpad"] == update_data["scratchpad"]