feat: add PATCH endpoint for session scratchpad

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

backend/tests/test_sessions.py

@@ -363,3 +363,101 @@ class TestSessions:
         assert response.status_code == 200
         data = response.json()
         assert data["scratchpad"] == update_data["scratchpad"]
+
+    @pytest.mark.asyncio
+    async def test_patch_scratchpad(
+        self, client: AsyncClient, auth_headers: dict, test_tree: dict
+    ):
+        """Test the dedicated PATCH scratchpad 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"]
+
+        # Patch scratchpad
+        response = await client.patch(
+            f"/api/v1/sessions/{session_id}/scratchpad",
+            json={"scratchpad": "- IP: 10.0.0.1\n- User: jsmith"},
+            headers=auth_headers
+        )
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["scratchpad"] == "- IP: 10.0.0.1\n- User: jsmith"
+
+    @pytest.mark.asyncio
+    async def test_patch_scratchpad_not_found(
+        self, client: AsyncClient, auth_headers: dict
+    ):
+        """Test PATCH scratchpad with invalid session ID."""
+        import uuid
+        fake_id = str(uuid.uuid4())
+
+        response = await client.patch(
+            f"/api/v1/sessions/{fake_id}/scratchpad",
+            json={"scratchpad": "test"},
+            headers=auth_headers
+        )
+
+        assert response.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_patch_scratchpad_empty_string(
+        self, client: AsyncClient, auth_headers: dict, test_tree: dict
+    ):
+        """Test PATCH scratchpad with empty string (clear scratchpad)."""
+        # Create session and set scratchpad
+        create_response = await client.post(
+            "/api/v1/sessions",
+            json={"tree_id": test_tree["id"]},
+            headers=auth_headers
+        )
+        session_id = create_response.json()["id"]
+
+        # Set scratchpad
+        await client.patch(
+            f"/api/v1/sessions/{session_id}/scratchpad",
+            json={"scratchpad": "some notes"},
+            headers=auth_headers
+        )
+
+        # Clear scratchpad
+        response = await client.patch(
+            f"/api/v1/sessions/{session_id}/scratchpad",
+            json={"scratchpad": ""},
+            headers=auth_headers
+        )
+
+        assert response.status_code == 200
+        assert response.json()["scratchpad"] == ""
+
+    @pytest.mark.asyncio
+    async def test_patch_scratchpad_completed_session(
+        self, client: AsyncClient, auth_headers: dict, test_tree: dict
+    ):
+        """Test that scratchpad can still be updated on completed sessions."""
+        # Create and complete session
+        create_response = await client.post(
+            "/api/v1/sessions",
+            json={"tree_id": test_tree["id"]},
+            headers=auth_headers
+        )
+        session_id = create_response.json()["id"]
+
+        await client.post(
+            f"/api/v1/sessions/{session_id}/complete",
+            headers=auth_headers
+        )
+
+        # Should still be able to update scratchpad on completed sessions
+        response = await client.patch(
+            f"/api/v1/sessions/{session_id}/scratchpad",
+            json={"scratchpad": "post-resolution notes"},
+            headers=auth_headers
+        )
+
+        assert response.status_code == 200
+        assert response.json()["scratchpad"] == "post-resolution notes"